That and hard hitters/stunners who can abuse him in the level 1-5 game before reactive gets good. 5 or even 10 stacks is not enough to tank 2 heroes and some creeps pounding on you. As far as breaks etc. to disable it, by the time aghs sd, aghs doom or silver edges are popping up, reactive armor shouldn't be much of a problem. At that point a decent bloodstone/other items and his mobility keeps him alive more than a little bit of regen (and everyone hits harder at that point).
I was laning against a silencer + slardar earlier as timber and it sucked. Silencer q, slardar run up and stun, silencer e = I got shit on hard.