(I don't have a UHK and though I looked before I don't think it's for me, just saying.)
The function keys are a bit more of a pain, but once again it's a thumb press + number (1->F1 etc). I'm more inconvenienced by eclipse and visual studio not having the same button for debug-step-over than actually pushing the thing.
I'd miss FX keys too, I use them a lot in window manager desktop switching.
Despite being two keypresses, this is usually faster to type, or at the least to return to home position from, and those keys are probably more consistently placed than the ESC key between vendors / models.
Although I'd also recommend making Esc available on a thumb cluster.
ctrl [ works everywhere vim is installed - no customisation required at all.