If Tracer were at all in the meta people would be singing Sombra's praise right now (in terms of her being a powerful character). Its exactly the same skill to aim Sombra's guns as it is Tracer's; just gotta keep the cursor on the enemy no matter how much both parties are dodging like madmen. That skill has been forgotten tho because, well, Tracer's been forgotten for the most part.
The difference being that there is no range at which Tracer's guns beat Sombra's guns. She's about like 76. She trades reliable headshots, no rockets, and generally lower damage for actually being a tank killer somehow. Between the self heal and the almost-exactly-tank-sized-at-mid-range spread, its hilarious to watch squishy flanker Sombra absolutely shit on Winston. How the mighty have fallen.
The reason people use Sombra from their own lines is because her flanker tools are kind of... purposeless. She has *two* abilities that are canceled if a D'va so much as wings you for one damage, which is just a super awkward mechanic for much the same reason Symm's turrets are awkward (and likely to get altered in her rework). Sombra's kit gives her amazing ability to get behind enemy lines but once she gets out there her main tool to survive getting ganged up on is her teleport. Not only is it awkward to use but it can have, at maximum, a 23 second cooldown. Considering the highest cooldown the other flankers have is 8 seconds, and... yeah. You can harass the enemy healer but if they're running a meta 2 tank + 2 healer setup, it ain't going to do much alone (especially compared to Genji who can straight up explode squish characters in what is effectively one attack, even after the combo nerf). Torb's buff isn't doing Sombra any favors. The hack is great except that against a good team its unlikely to be successfully applied except via EMP.
So you have this character that's amazing at getting behind the enemy but pretty gimmicky once she's there, competing for Genji's slot on the team. And more broadly, the 1-3 attack/defense slots on the team. Of course high level players aren't crazy for her. I would say in general Sombra's gun is over the top but her kit just doesn't quite mesh into a reliable playstyle. Having to channel her hack in sight of an enemy, combined with the fact that Rein can spin like a top to break it, seems plenty a weakness for her hack; having it be broken by damage basically means it doesn't exist at high level play. I'd say nix hack cancel on damage (perhaps alongside a nerf to range or channeling time), and nerf her gun. If her gun is to be fair it should be shorter range and/or lower damage in general. I'd also say the transmitter needs to be... I don't know, its awkward, something about it needs to change but I'm not sure what. Too easy to use to escape, takes her out of the fight for too long when its used. Its almost like Sombra is a dead ringer spy.