After 10+ years on my old cheese grater and firewire interfaces, it was time for a new system. I went with a MacBookPro and MOTU 828es, but after I encountered two units with flaws I traded "up" to an Apollo x6, which turned out to be a disappointment. Other Thunderbolt interfaces like Lynx and Focusrite were >$3K, more than I wanted to spend.

USB
I was not looking at USB interfaces because of bad experiences in the past, but a subsection of this forum praised RME for its reliable drivers and I decided to give it a shot.
Long story short, USB has not been an issue for the last year and a half, I leave the buffer in Cubase/Logic/GigPerformer at 128 and it's been solid.

SOUND
To my ears, sound quality is really good, I prefer it to the Apollo. Then again, I prefer the MOTU to the Apollo as well, so my opinion is certainly not the majority view.
The mic preamps are great, too. I sold my Daking Mic Pre as it didn't really sound any better than the on-board ones. Short of "character" pres, I wouldn't bother with externals and you can confidently record with just this box. 75 dB gain is available, sufficient for most mics.

FUNCTIONALITY
Connectivity is decent for such a tiny box and essentially sufficient for my home setup, but sometimes I do wish I had spent more for the UFX or 802, which compare more directly with the MOTU 828es, albeit at a much higher price.
TotalMix is great, very flexible and deep, but easy to understand and use. RME uses some idiosyncrasies which may appear odd at first, but it's not illogical and you'll get used to it.

EASE OF USE
As for ease of use, after one week with the UCXII, I caved and bought the ARC remote, which is a must-have in my opinion. They simply can't fit all the controls on that 1U half rack front panel and even though controlling it with TotalMix is perfectly fine, for everyday use you're gonna want to have that dial and buttons.

COMPROMISES
The UCXII's size dictates some understandable design compromises, no XLR outs, and a wall-wart PSU. At least the latter comes with a locking plug, but an odd and indefensible choice to me is the Type B USB connector. For a product that was released in 2021, USB-C would have been the correct decision.

CONCLUSION
All in all this has been a great purchase, and I actually like its compactness as it makes it possible to have studio-quality gear in a backpack. It sounds great, it's flexible, reliable and portable.
At $1699, it's far from cheap, you can get more connectivity for less cash. But I feel the UCXII is money well spent.
Finally, RME has proven to support their products ad infinitum. It's reassuring to know they still provide updates for twenty-year old PCI cards, and they actively troubleshoot with customers on their forum. Recommended.