That was the inital reaction to her but people are starting to realize some of her negatives. When it comes to "in your face killing shit" Tracer is still way, way better than Sombra. Without any reliable in-the-moment escape and movement skill Sombra is very easy to take out. She excels when she can plant her E somewhere, dive and E out. If anything I feel she plays a bit more like Reaper than Tracer.
But she does way way way less damage than Reaper. Honestly, I'm not sure she's that good, fun sure, but good, eh. She's definitely situationally useful especially in coordinated pushes and stuff, but she seems to kinda lack killing power/sustained harassment that Tracer can do and she definitely can't clean out the room like a Reaper can.
She's very utility oriented, she counters some heroes decently(especially Tracer, Torb, Bastion but those heroes aren't like stuff you'd specifically want to counter with a special pick I'd say) but don't know how much pressure she can really put on a team by harassing, her damage is nothing crazy and if you focus her, she has to TP out of a fight but unlike Tracer who can kinda just hang around and keep being a little annoying bitch, once you TP out, you have to reset your stuff, invis in, wait for tp cd, throw a tp, then attack, meanwhile whatever harass you did is probably healed up. And with damage that's not outstanding, it puts a lot of focus on her utility, which I'm not sure is good enough to be worth a spot.
Guess we'll see, Mei had the same issue and early on people would never play Mei cause why play Mei and do freezing things when you could pick another damage hero and just kill things, but people got used to her stuff and started really using it efficiently to make up for her damage. Also they increased her damage, so that helped. But that's another interesting question, would you pick Sombra over Mei if you wanted utility/team fight disruption instead of damage?