I find these statements to be a bit confusing. When one is working in ACR, he/she is really dealing with three color spaces: the raw space, which is converted to the internal working Melissa space and then to the ACR working space such as ProPhotoRGB. The only RGB numbers that are accessible are the final working space numbers. The destination workspace must be taken into consideration when interpreting the RGB output numbers.
As to the three possible color spaces, I'm not the least bit interested in the so called "Raw color space" since that's not provided to me in either product. And I don't want to go down that endless rabbit hole about "does Raw have a color space". That leaves ProPhoto RGB linear TRC or ProPhoto 2.2 (Melissa) the latter is the only set of numbers I'm provided in LR. For the task of working skin tones numerically, doesn't matter.
ACR provides the 0-255 values based on the encoding color space you select in workflow options. Nice because you now see the numbers you'll get in Photoshop. Not as nice for this topic in terms of not having percentages (which make working numerically with skin really easy). Also not so great because its not on parity with Lightroom (and vise versa).
LR provides you those nice percentages. It is based on Melissa RGB, the differences between it and the actual Raw processing color space being simply the TRC (2.2 instead of 1.0). Again, very useful in this context for numeric skintone work due to the percentages.
So we have two identical Raw processing pipelines. Each has an advantage in terms of the numbers provided, neither provides BOTH advantages (which would be really, really nice).
Advantage ACR: numbers match encoding color space you end up with in Photoshop. But ACR expects you to select this from the get-go and will provided a rendered image as soon as you're done. LR doesn't work this way, it doesn't know when (or if) you'll render the data into Photoshop (you may never, you might print directly, or upload a web gallery without ever opening Photoshop).
Advantage LR: percentages while working. Too bad we can't now use the same percentages once we're in Photoshop but alas, the RGB info palette doesn't allow this.
In both ACR and LR, you're pretty much expected to do as much global tone and color work as possible for reasons that should be obvious to anyone here. In a prefect world, both product would allow us to toggle the scale.
Note that you can build a Melissa RGB working space in Photoshop's custom RGB color settings. Then you could take a Macbeth LAB generated doc, convert it to ProPhoto RGB and then Melissa RGB. You'd see that while the numbers are of course different, the ratio's are nearly the same (within a few values).
In the grand scheme, all three varieties of "Pro Photo" all work equally well for this discussion, assuming you get percentages because those percentages vary a tiny amount. Melissa RGB is kind of useless compared to the other's IMHO because its not based on anything "real" but its close enough. I'd prefer if we'd just get a 1.8 TRC and just call it ProPhoto RGB or even better, just show us the 1.0 TRC values. But the chromaticity values are the same.
So I wish Adobe would allow ACR and LR to share the same processing color space using the same scales as an option, then allow Photoshop users to do the same (add percentages based on the working space to the info palette). But in the end, short of not providing absolute exacting values, LR.'s percentages make correcting skin tone numerically very easy. Personally, I'd prefer to just LOOK at the image on a calibrated display. But I recognize some users just have to have a numeric road map. I think that map should be spelled out using something that at least has some basic hook to the processing color space. In LR that's far, far closer than some arbitrary CMYK color space by a mile.