The sombra problem can be summed up in one sentence:
Why get entangled with a character who has to deploy so much effort to kill any target while other characters kill their targets much more easily.
The whole problem is there.
When you play Sombra too many elements of its gameplay are there to prevent you from being really effective so it discourages you quickly enough.
The duration of the stealth is too short.
The weapon’s Dps is crappy.
The mechanics that forbid us to start hacking or shooting under stealth (automatic stealth release) breaks the rhythm of Sombra’s game.
I know that Blizzard does not want to make us murderers, but that’s exactly what the character is missing.
A real ability to kill … in an FPS it seems not unrealistic as demand.