So for AI units and such, you’d probably want to have each AI unit write its position to the blackboard (haven’t looked into how the blackboard works in UE4, but if it’s like standard ones, this should be how to use it), instead of using a physics system to query, unless you’re actually doing something like avoidance, in which case where overlap triggers and stuff are good.
But for actually querying, blackboard is the best, and it doesn’t cost too much (shouldn’t anyway) just to set a position onto that each tick. Then your AI can access it.
If you need to make them move together, or do avoidance, a position and knowing a radius for a sphere that covers the unit is pretty dang good, and also quite lightweight.
If you’re doing something different of course and need like events for overlapping, that’s when it’s nice to use physics volumes for triggers and such.