I'll take the opposite tact and say I don't feel like I see enough POTGs from these heroes:
- Winston -- I've seen exactly one Winston POTG, and it was back on the beta weekend. I got a couple of environment kills (which are weighted too heavily, IMO) with my ult on Nepal, I think it is? Because his ultimate isn't particularly dangerous outside of contrived scenarios like that and his hit-and-run playstyle outside of the ult, he is hard to get one with.
- Lucio -- As somebody else mentioned, the only Lucio POTGs I've seen are based around environment kills. Those are fun, and sometimes even deserving, but I have literally not seen a POTG of him doing his primary job. And I include in that things like absorbing an enemy ult with his own.
- Symmetra -- I actually did just get one for a 3-4 kill (D.Va Mech + D.Va, Mercy, Soldier, however you choose to score that) with my regular attack that was pretty awesome. However I've only seen one other POTG from her in my time playing.
- Zenyatta -- Haven't seen one in live yet, but he's also the least-played hero in my playing experience. I think I saw a couple in a beta weekend. Do I remember reading recently that they changed POTG code because it's not interesting to watch him standing around glowing? I don't disagree, but it may have gone too far.
- D.Va -- I can't recall if I've ever seen one from her. Her ultimate, while it's good tactically for forcing an enemy team to scatter, is too easy to avoid 95% of the time to get the kind of big kills that show up on POTGs. Her guns can be super deadly up close but if she's that close there's a fair chance she's not going to survive long enough to chain kills, and at range, the POTG couldn't be anything more than her stealing 10-20 point kills because the damage fall-off is high and the POTG window is short.
I think most of the others are in a fine spot, with maybe Reinhardt and Tracer on the low end and Mercy and Torbjorn on the high end. With Torb, I think all they need to do is cut the contribution toward POTG from his turrets; if the actual player is going ham--and he's certainly powerful enough to do it--they by all means give him POTG. But, for example, a few days ago when I was playing with a friend and literally all he did for his POTG was stand there smashing on his turret while it did the work isn't something deserving of POTG for me. I do like that they try to give credit to people who interrupt enemy ultimates though, even if the play sometimes isn't terribly interesting in the end.
I think the #1 change they could make to the POTG system is to make the window (and replay) a little bit longer. That would help get more plays that aren't just ultimates into the picture more consistently and open it up to more heroes at the same time. And for ults that don't just happen instantly (Genji for example) it may make it even more epic by including kills it wouldn't have.