Having sentry classes is not the most fun mechanic, but there's a lot of frustrating shit in this game (roadhog stun combo/mcree combo/snipers/general genji bullshit). I don't find turret teams too bad to fight with more experienced players, even in pugs. Trollborn's turret explodes fast under any sort of pressure, bastion is an easy pick for genji or roadhog, and symmetra's sentrys are made of glass and she's susceptible to dying before getting tele down and essentially being useless/poor when focused down and team is behind. These feel fair enough to me though if your team is new then these classes are absolute noobslayers, and I agree its very frustrating to just watch your team mates derp in one by one getting picked off by a stationary turret instead of just shooting the fucking thing.
Widows on the other hand ... we played against 3 decent widows on defense last night, with insta head shots coming in three directions it was a very anti-fun and frustrating game, even winstons were just getting annihilated before getting close, even if we got one down their run back was obviously shorter and we were dying in the process. Having played for quite a while now and watching the tournament at the weekend, I'd be happy if blizzard went with no class stacking at all. Makes it easier for them to balance the game, still allows swapping of character during game for some element of the game involving class-counter strategy, but avoids turning the game into a 'oh you're running 4 winstons guess we better run 4 reapers' type of shitfest with strategy mainly focused on countering stacks rather than general ability and ultimate use & cooldown.