Thumb shift keys are definitely an (incremental) improvement over the traditional placement, but they require a yet bigger commitment to retraining from the standard keyboard layout. I've been using Kinesis keyboards for 25 years, and finally this year committed to moving the shifts to the thumbs. This was worthwhile for me, but I hesitate to recommend the, ahem, shift to others.
why not both? despite having it mapped there for 20+ years I still don't use the thumb-shift. but for some chords its really a lot nicer.
The shift keys take up precious thumb cluster space, and their standard location is also useful. IMO the "best of both worlds" option is just worse.
The most puzzling default is that the 'up' key is on the left and 'down' is on the right. This is the opposite of vi and ctrl-J ctrl-K for no reason that I can imagine. I always remap it.
Hm, Ergodox don't either, at least in the suggested default layout.
I think they want to keep it somewhat approachable..