Having a good keyboard is important (not only for ergonomics, but also for efficiency) in our business. But I find I'm having difficulty when temporarily switching from US to DE keyboard layout, or when using a mac (vs linux) - so what would using a completely custom keyboard do to my general computer user ability?
This is why I never considered buying one of these...
I find that changing the layout creates significant mental overhead for me as well, so I just don't do it. I use the NEO layout [1] which contains all the characters and symbols I could possibly need in reasonable locations. I barely need to leave the home row to write any of the symbols needed during programming or writing TeX.
As for physically different keyboards, I've found it to be a much lesser issue than I expected. I recently got a Moonlander [2] due to general hand and wrist pain that could only be attributed to strain from typing.
I took care to map the layout quite closely and the physical layout was almost immediately usable. I started at around 2/3 my normal typing speed but was back to 100% in a matter of a week.
Going back to a normal keyboard takes minimal effort. I can still type on them quickly, I just loathe them more than I did before now that I know how much more comfortable it could be.
At least on the mac I do not use the german layout at all. For typing an Umlaut I type instead Opt(Alt) + 'u' to get an ¨-sign, then I type a/o/u to get the letter. The same with ß - just Opt + 's'.
It is maybe a bit annyoing if I have to type in german a lot on some days, but it is less annoying then to keep an extra layout. And since I sometimes have to type in french and often in russian... well, you get the idea, it would be a mess of 4 keyboard layouts. With special characters I can push the amount of keyboard layouts just to colemak and russian.
This is why I never considered buying one of these...