assist is a little flaky. I dont quite have the problems you are stating but every once in a while my slaves will get stuck targeting a dead mob that I looted or something. No amount of /assist or spam will get them to target the mob thats beating on the tank. I have to esc out and clear the target before it will work properly.

Personally I think this is an issue with using /focus. If I use my tanks name instead of making him the focus I don't have this problem.