When I tried raging while I was webbed/entangled by those invisible webs that can appear as you walk it said that i could not concentrate so the rage did not work.
It isn't just custom rages. It is all rages in general. Rage counts as a spell-like ability and the Entangle effect will interrupt it just like it would a spell.
That makes no sense to me.
Yeah. That's been around. It's bioware default and annoying and makes no real sense, but not related to custom rages.