I was told by one friend that they made it really hard to use a normal keyboard effectively after a few years. Did you find that?

It really depends on a keyboard - if it is similar to regular one or not. The UHK is pretty similar to regular keyboard - keys are staggered (not ortholinear), there are no additional thumb keys which are also not available on a regular laptop keyboard. After a year with UHK I'm still able to jump between laptop and UHK when needed. Somehow muscle memory "detects" when my hands are on UHK and I start using additional shortcuts which are not available on laptop.

Meanwhile if you'd pick something like Dactyl https://github.com/adereth/dactyl-keyboard, I believe it would be more difficult to jump arround.

P.S. After a year my typing speed on split keyboard is higher compared to laptop.