Not just noob tubes. Explosives in general. More than 50% of deaths in co-op are to grenade launchers. Very often the AI will just fire a barrage of grenades shooting at the same time, all 5 of them. Sometimes it feels like a single AI with a grenade launcher is able to fire 5 grenades at the same time.
The very first thing that needs to be addressed is grenade safety mechanism. They are not supposed to explode until they fly a couple of meters. Right now an enemy AI just strolls into a room, get's shot 3 times and is 1 bullet away from death but manages to fire a grenade killing everyone inside including themselves.
They act exactly like suicide bombers, except with an instant explosion making them a hundred times more effective.