Okay so after playing Sombra a lot on the PTR, I've learned a few things and I'm just going to give a heads up for everyone trying her out for the first time.
Even though Sombra is on offense, she really has low DPS compared to other offense characters. I mean, yes her SMG does good damage but you really can't 1v1 people. Even though Sombra has the most abilities in total (4 including her passive), none of them do damage! You main job is to disrupt the enemy, kind of like Tracer, but your more annoying when played right. You should never be on the front lines, because you are weak there! Be behind the enemy team. Hack healthpacks near areas where fights usually happen, people forget that she can hack healthpacks.
Also, if you were near a hacked healthpack and you were fighting someone 1v1 or even 1v2, you're gonna win, so its really fun. If you were fighting a sombra near an enemy hacked healthpack, leave her alone
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You counter: Zarya, Widowmaker, Reinhardt, Bastion, Torbjorn (if your team is on point).
You are very weak against: Mei and Winston. NEVER be close to Mei, ever (unless you have a translocator somewhere). She's ridiculously strong against you and it's not even funny.
Her ult is a game changer, especially used as a clutch in the end or overtime. It's LINE OF SIGHT, not area of effect, that means you have to SEE someone to effect them (of course they have to be close), so you don't want to jump in the middle with enemies behind you and then ult, it won't effect them.