> If I said a microcontroller felt pretty army, that's different to saying it feels arm-y, since army is already a word.
I'm assuming that you don't mean that the microcontroller reminds you of an upper body appendage, and are instead referring to a well-known instruction set architecture. If that's the case, I'd recommend 'ARMy'. But you're correct about the ambiguity where the word form you want to use collides with another pre-existing word. In that situation, you'll naturally need a different solution.
> Language is meant to convey meaning, and you understood the meaning I was trying to convey, so what's the issue?
That's precisely it. Language is meant to convey meaning: and the more complex and abstract the meaning we mean to convey, the more precise we need to be in our use of language. Deviating too much from well-established conventions makes ambiguity or miscommunication more likely. Conversely, to your point, blindly adhering to conventions where that would increase ambiguity also impedes communication.
In this case, the erroneous hyphen didn't completely erase your intended meeting, but it did make the sentence slower to parse.