Zynthian is pretty portable as far as code goes, it’s more design decisions that are off putting.
Zynthian expects everything to run as root and has a fairly opinionated view of where things should go in a filesystem. I’d never recommend running it alongside a desktop or mobile OS. Otherwise, go for it!
I had a very brief try of running softsynths on a Pinephone and it worked. I bought an early model that OTG doesn’t work on though.