this week I implemented this behaviour tree (inspired by migration to UE4), replacing the old state machine, and I'm quite happy now. sucked means stucked code is more simple, thus performance is slightly better.

basic description: the group manager defines desired targets, and units start to move if it differs from their position. sounds simple

conclusion: it is better to use a clean easy-to-debug system for AI design; and without visual tools it is hard to make a complex one (like in UE4).