I like behaviour tree, although the first time was a hell XXD, my first AI was a zombie type where the alien goes where you are and hits you and I saw everything perfect but …
Now I have started the AI of a drone, he needs a behavior to move and shoot and I have realized how rare it is for me to put both things on the same behavior tree.
Each one has a different “mission” with completely different tasks and decorators.
Being two such different behaviors it would be much more practical to have two separate behaviours tree.
Being small would be more manageable to work and as MieszkoZ says:
"And a bunch of simpler BTs will always be better CPU-performance-wise than one big & complex BT. Also, having many BT assets is no problem at all in terms of memory, so I’d recommend going that route. "
One AI mission = One behavior tree