Genji is also just "Tracer-with-a-twist". You have to remember that bad players will feel that similar characters are much more different than they are and Blizzard games are designed for bad players.
Sombra is aggressively situational. Tracer is good to be a decoy, just constantly bolting around, distracting a player long enough to force them to engage her or suffer the consequences.
Sombra just suffers from being 'too technical' where she basically is for the toughest setups.
She cant go invisible in the middle of a fire fight though (it breaks when hit) and needs to pre-emptively have her grenade ready. If she tries to turn around and aim the grenade in a game like overwatch, she'll be dead while its still flying through the air.