Thumb arthritis: I've started to develop arthritis in my thumbs when grasping, and the EZ doesn't trigger it at all.
Effort to learn: I wouldn't say "serious dedication", but it did take an effort. I had a few bad habits, namely pressing 'b' with the wrong hand (can't do that on a split), and having to look when typing the numbers. It took me a couple days of feeling like I couldn't type, and maybe two weeks before I felt like I was 100% comfortable with it.
I do agree that the thumb clusters, other than the two primary keys, are basically useless. I have some VERY infrequently used keys mapped to them (pg up/down, alt). It also does take a lot of space to use and to carry.
I use a customized keyboard layout particular to my workflow with i3 and vim. I'm SUPER happy with it.
I've been very tempted to try the Moonlander, but I also really don't want to go back to labeled keys. My current keys are blank. If they had a blank set of caps, I'd have ordered it.