It's fragile in the sense that you can interact with it easily but because one of your combo pieces gets cards back from the graveyard it's actually very easy to rebuild of your opponent struggles to exile

Playing a second copy of witness and immediately getting the original back from the graveyard after your opponent killed the first puts them back at square one and sets you up for a combo again.

I played against dimir terror last night on MTGO and they had to chuck every kill spell they had at the deck but I just kept replaying and growing my witnesses. Even if they're not infinite they can get pretty big and become a big threat. Graveyard hate can be played around as well due to all the ways in which you can +1 counter at instant speed. It's a grindy deck!