"I don't mean to get the year of to a start with a bun fight but I would have to 100% disagree on the GCR with UCA vs UCR, and yes it is what I do for my "day job". With over 10 years prepress experience I have lost count of the number of people I have come across tearing their hair out trying to convert digital shots to CMYK using UCR. The fact of the matter is that when scanning film, more often than not the conversion was done using the drum scanning software, perhaps Linocolor, or another of it's ilk."
Don't want to beat a dead horse here, but there simply is no difference in separating film scans or digital captures. If there were, I would have seen it thousands of times. If you want to argue methods of conversion, that's an entirely different story, and if you've been involved in prepress for 10 years (as have I) and are still using Photoshop's servicable but lacking classic engine, then perhaps I can understand your concern. The differences you allude to have nothing to do with the source of the image, only the tools used for conversion.
Drum scanners capture in RGB, and while the scan was traditionally converted on the fly in the scanning software, most of us now capture hi-bit profiled RGB and leave the conversion for later depending on the destination of the file. Photoshop now defaults not to the classic engine, but to the Adobe CMM and whatever profile you have loaded in your color settings. The quality of conversions in Photoshop using the Adobe CMM and custom profiles is easily equal and in most cases superior to the on-the-fly conversions that pre-press scanner operators used to make.
"The UCR conversion in Photoshop is very harsh and leads to a tonal break in the 3/4 to shadow transition. The main cause of this is the removal of too much CMY at the transition (think grey shadows under chins instead of the skin tone that was there in RGB) . Thankfully most of the defaults in PSCS2 are now GCR, which allows the control of UCA."
No one I know recommends using the UCR option in the classic engine. If you're going to use the classic engine, light GCR with no UCA is usually recommended, but we have better tools now - custom and canned icc profiles.
"You have to remember that the main (but not only) purpose of UCR & GCR is to reduce printing cost."
Mostly to be able to control the ink limits on web presses to allow them to run faster and to still have a convincing black.
"Also on the point of using Epsons for proofing. It is all in the software."
You don't need any special software to to great pre-press simulations. All you need are good profiles for your pre-press proofing system and for your Epson. You can easily do the rest in Photoshop.
"Without exception (OK maybe the odd one who can afford Kodak Approval) every prepress house and publisher I deal with uses Epson or Agfa Sherpa (re-badged Epson) for proofing with a rip and press profiles."
Kodak Approvals, DuPont Waterproofs, Fuji FinalProofs, Imation Digital MatchPrints, are all pretty standard digital proofing systems these days. In my experience, it's the odd one who doesn't use one of the above. Sherpas and Oris proofing systems are pretty good, but not my favorite, but you have to deal with whatever your vendor uses.