What can ACR do that LR can't?

[texshooter]

Is there anything that Adobe Camera Raw can do that Lightroom Classic cannot?

[digitaldog]

More like the other way around.
If on version parity, offhand, I can't think of anything ACR can do that LR can't other than how a raw can be hosted; ACR can host Bridge and Photoshop, LR uses it's DAM/Catalog.

[fdisilvestro]

When using "Edit in" an external editor, LR allows only 4 ICC profiles to chose from (Prophoto RGB, AdobeRGB(1988), sRGB or Display P3), and the histogram is based on the internal working profile regardless of what you choose.
If you want to output files using any another profile, you have to export first, and still you can select only RGB profiles.

In ACR you can choose any ICC profile, and the histogram reflects the output in that profile (yes, it will show CMYK values for CMYK profiles). This can be a difference if you use profiles such as CMYK or the Joseph Holmes ones)

[digitaldog]

When using "Edit in" an external editor, LR allows only 4 ICC profiles to chose from (Prophoto RGB, AdobeRGB(1988), sRGB or Display P3), and the histogram is based on the internal working profile regardless of what you choose.

When you use the Edit in Photoshop command in LR, ACR is doing all the raw rendering with the four profile options.
I'm not sure how any raw file can be processed otherwise from any 'external editor' since only LR and ACR understand the proprietary parametric raw edits. With non-raw files, well they are already in a rendered color space.
No limitations or a difference with ACR in reality.

In ACR you can choose any ICC profile, and the histogram reflects the output in that profile

Same in LR; just soft proof (type S key) and pick the profile you wish. The Histogram and numbers now reflect that profile.
No limitations or a difference with ACR in reality.

[fdisilvestro]

Same in LR; just soft proof (type S key) and pick the profile you wish. The Histogram and numbers now reflect that profile.
No limitations or a difference with ACR in reality.

Please explain how to softproof to an CMYK profile in LR or export an image with an CYMK profile

[digitaldog]

Please explain how to softproof to an CMYK profile in LR or export an image with an CYMK profile

There is no CMYK anything in the ACR engine that LR uses.
There were older versions of LR that did soft proof and convert to CMYK, they were removed (for good reason).
There is CMYK in Photoshop, however, which can convert to CMYK from ACR.
I have no idea why anyone would even think of doing this directly from raw data to CMYK and neither did the LR team (for good reason).
Hopefully, you'll note that in ACR, there is no CMYK numbers either but sure, soft proof. 

Edit In External editor doesn't limit any RGB conversions unless you apply a parametric edit in LR. You can open an RGB document in an RGB color space that isn't listed in the preferences so, if its in ColorMatch RGB and you simply ask to open in that external editor, LR simply hands of ColorMatch RGB. Unlike what you suggested, when using "Edit in" an external editor, LR allows any RGB profiles to be honored UNLESS you apply proprietary, parametric edits just like it would with raw.

[fdisilvestro]

in ACR, there is no CMYK numbers either but sure, soft proof. 

See attached image

Now, try to open a raw file as a smart object in PS from LR with a profile different than Prophoto RGB, Adobe RGB(1988), sRGB or Display P3.

[digitaldog]

Now, try to open a raw file as a smart object in PS from LR with a profile different than Prophoto RGB, Adobe RGB(1988), sRGB or Display P3.

There is CMYK in Photoshop, however, which can convert to CMYK from ACR.
I have no idea why anyone would even think of doing this directly from raw data to CMYK and neither did the LR team (for good reason).

There were older versions of LR that did soft proof and convert to CMYK, they were removed (for good reason).

[fdisilvestro]

Is there anything that Adobe Camera Raw can do that Lightroom Classic cannot?

This was the original question. There is something that can be done in ACR that cannot be done in LR. Maybe it is wrong, or not useful.
I rest my case

[digitaldog]

Is there anything that Adobe Camera Raw can do that Lightroom Classic cannot?

This was the original question. There is something that can be done in ACR that cannot be done in LR. Maybe it is wrong, or not useful.
I rest my case

Not much of a case; ACR can open a raw without having to import it into a catalog first.
One can host ACR and Bridge at the same time; can't do that in LR either. BFD.
Now, want to start a list of all the actually useful functionality LR provides ACR doesn't? It's far longer than the one's you and I have come up! There is lots that can be done in LR that cannot be done in ACR. But the OP didn't ask what he can't do in ACR he could do in LR.
I rest my case.

[texshooter]

I decided to join the herd and learn Lightroom. I'm sure it's come a long way since 2.0. I just wanted to make sure I won't be limited functionally with respect to smart objects, color spaces, interacting with Photoshop, speed of use, etc.

[Arlen]

For my use, the History panel in LR's Develop module is the one huge advantage over ACR. I still use ACR quite a bit, especially as a filter, but I find the lack of a history list there--to easily go back and forth between edit steps--to be a big disadvantage. I wish they would implement that in ACR.

[jrsforums]

For my use, the History panel in LR's Develop module is the one huge advantage over ACR. I still use ACR quite a bit, especially as a filter, but I find the lack of a history list there--to easily go back and forth between edit steps--to be a big disadvantage. I wish they would implement that in ACR.

I believe (could be wrong) the history data is stored in the LR catalog, not metadata.  If that’s the case, be difficult to implement in ACR

[digitaldog]

I believe (could be wrong) the history data is stored in the LR catalog, not metadata.  If that’s the case, be difficult to implement in ACR

You are correct on both. Ditto for Virtual copies and Proof copies; but tremendously useful for LR users that ACR can't provide.

[Arlen]

but tremendously useful for LR users that ACR can't provide.

A really huge advantage. Whenever I'm processing something in ACR, I feel like I have one hand tied behind my back.

[texshooter]

I'm liking Lightroom so far. I found this tutorial very informative.
https://www.creativelive.com/class/advanced-lightroom-workflow-jared-platt

I'm still confused about one thing though.
Lightroom allows the editing of Smart Previews, which are tiny compressed DNGs, using all the tools in the Develop module (except for roundtripping to Photoshop). But my gut tells me this can't be possible without losing some edit accuracy/quality. How is it possible to edit highly compressed DNGs and still get the same quality results as editing uncompressed RAWs?  I understand Lightroom is not editing pixels but rather building a set of instructions, but Lightroom relies on the data in the RAW or DNG file for the information upon which it builds those instructions.  The less RAW data there is to work with, the lower the quality edits you end up with, is my guess.  So how is it possible for Lightroom to edit Smart Preview without paying a price somehow?

[digitaldog]

See: https://helpx.adobe.com/lightroom-classic/help/lightroom-smart-previews.html
Specifically this about Smart Previews and where they are useful:
Continue to work with your Smart Preview files even when the device containing your original photographs is disconnected. You can perform all edits that you would perform on the original file.

Once created, your Smart Preview files are always up to date. When your storage device is connected, any edits you make to the original file are applied instantly to the Smart Previews as well.
IOW, it's a preview. If you're not going to be going 'off line', you may not even need to consider them.

[texshooter]

Thanks, but those web articles simply reiterated what I already knew, namely that Lightroom allows the editing of compressed versions ("Smart Previews") of the original RAW files without having to be connected to the drive containing those original RAW files.
What I'm scratching my head over is how is it possible for Lightroom to do this without losing editing accuracy.  The Smart Preview DNG is only 3% the size of the original RAW file out of the camera. Intuition tells me if I edit these itty-bitty DNGs, my edits must suffer someway somehow.

[digitaldog]

They are just previews and the actual edits are then applied TO THE ORIGINAL data when you reconnect. Nothing lost.
Everything LR does (and ACR) is parametric editing. Instructions. The instructions can (and are) used when you render the original data. There should be no data loss.

[texshooter]

Here is a statement I read that confirms my suspicions all along.

"For accurate detail work like Sharpening and Noise Reduction, the 1:1 on a Smart Preview is not the same as the 1:1 on the original uncompressed file. You’ll have to perform these types of adjustments once you have the original file available again. "

So I will never edit off of Smart Previews. Only original RAWs for me.