The big difference between the Ergo/Moonlander and UHK is ortho vs staggered layout. Ortho (rather, "ortholinear") is an increasing common layout where the keys are arranged in straight rows and columns. The jury is out on whether it's useful, but legions of OLKB fans swear by it. Also, UKB hasn't shipped yet, where the Ergodox has been out for years, and there's excellent support for flashing using QMK. The Moonlander just landed (heh) a few months ago, I haven't heard too much about it, except that finding custom keycaps for the thumb console is going to be hard/impossible. I guess that's the biggest hindrance of all these ergonomic keyboards: it's really hard to find custom keycaps.