I built (two) ErgoDox(en) and ended up using the Norman layout with various attempts at layers over the years. I'm now using a Lily58 with low-profile choc switches that I'm generally happy with.
I'm fine switching back to Qwerty keyboards, but if I were to go down the path again, I would advise myself to stick with Qwerty. I still have a pretty crazy keyboard layout and appreciate the ergonomic features.
Also, I've realized that everyone is different, and what works for one might not work for someone else.
Advice that I think works for everyone is to listen to your body. If your body is telling you that something isn't right, take steps to address them.
I switched because I never typed with proper technique in QWERTY. I would only use ~2 fingers on my right hand. I tried multiple times to retrain myself but there was just too much muscle memory. The only way for me to switch to proper technique was to switch layouts.
Fair, but how often are most people typing on someone else's computer? (Outside of developers who do pairing.)
At work there's less sharing but back when we were in the office, it happened. I also have two work provided laptops (one from my employer and one from client) which are heavily locked down.
Finally there's all the other keyboard manifestations in my life - on my work iphone and my personal android phone and the tablets and all the tv apps that pop up a keyboard to sign in etc.
Qwerty may be suboptimal but it's freaking everywhere.
Then there's the reverse: My PC at home - wife wants to check something quickly without turning on her PC.
Finally: I regularly use both a PC and a laptop. Depending on how weird you want the layout to get, there may not be a way to use it on the laptop. Schlepping an external keyboard around - nope, not practical.
The direct with family is similar - you can have an extra layout installed
So the leaves even fewer cases where you really have to switch (but unfortunately not 0, no easy mechanisms to just easily switch to a some downloadable layout named after you on the fly at any computer)
Meh. I can type fine on both my weird layout and on a regular one (either Swedish or English layout).
Your point about speed I agree with, but I'll say that the point of an alternative layout shouldn't be speed, it's about comfort.
The split key ortho plugged into my monitor, therefore, can be as weird as I want. It's heavily customized and I can do more with it, faster, easier. I do type about 15% faster flat-out with it, but more importantly, I can send λ with little more effort than `l`. But since the laptop has the default layout, I remain "bilingual", two independent muscle memories can be maintained pretty easily if one regularly uses both.
The ortho does have a QWERTY layout for the alphabetic keys, since I don't see the point in changing that, but I don't expect it would make a difference if it didn't.
Yep, you do type faster. In my case, it was 10%. Honestly, it wasn't worth it. There are way too many disadvantages, not least of which is the fact that that only your computer has your weird keyboard. Also, typing 10% faster is pretty irrelevant: how much of your time are you typing flat-out?
If you need an ergonomic keyboard, that's a different thing. There are lots of those. Get one that uses qwerty.