Doom and Hammond arent fundamentally bad in 5v5, they just have too high of a skill requirement for value compared to other tanks. In doomfists case, going off what I see from mirror and zbra, he plays like a more aggressive combination of zarya and winston. Go in, charge punch, go out. Go in, punch, secure kill with 2 shots (it’s almost always this exact routine). Alternatively, go in, punch enemy tank into your team, farm tank.
Problems arise really fast when you misread when to go in or when you can’t hit those 2 followup shots a very high %. Unlike Winston, block doesn’t guarantee safety for doomfist, and his followup mechanics are more demanding.
When people misread who to engage on and when, or miss followup shots, they feed. Then negative reinforcement conditions them to not do that, they frontline too much. Or worse, they are autopiloting and frontlining by default.
Probably lots of ways they could fix it. Make his frontline better, make his block safer, make his followup damage more reliable. Knowing Blizz, they’ll probably buff something obscure that gives him a tiny advantage in niche situations and call it a day. 