The difference is literally any other flank option is visible and you can hear footsteps/abilitys when they are gap closing/attempting to flank.
Sombra on the other hand decides 100% of the time whenever she wants to engage.which is usually when the teamfight already begins and the only time you can hear her attemping to flank is when she just tp’d relatively close to you.
No other hero forces you to play with 100% attention the entire match as long as the sombra didnt just recently die because you never know when shes gonna appear. Its also the only hero that requires 1 person to always be near the objective when defending because all other backcappers can be spotted attempting to do it.