[bregma]

Othar than word size (eg. i386 vs. x86_64), endianness (eg. PPC vs. x86), alignment (eg. ARM vs. x86) there's plenty of other low-level issues like parameter passing conventions, call frame handling, exception handling, presence or absence of optional instructions or registers, and a number of other subtle gotchas that could affect a port. Instruction probing is one of the ones we run into where everything builds find and fails at runtime.

If you're working at a high enough level you're probably fine. Like the difference between being an auto mechanic and using the app to hail a ride.

[smallpipe]

You're missing the biggest one, concurrency model.