The only reason Sigma ever gets to be played without downsides is when the meta is enforced hitscans and his counterpicks are trash.
Namely, the entire game’s meta since the start of double snipers/shields.
Widow/Ashe/Tracer → Widow/Ashe/Tracer/Soldier → Cassidy/Soldier/Ashe/Tracer → Sojourn/Sombra/Soldier/Tracer → Soldier/Cassidy/Tracer → Widow/Soldier/Tracer → Tracer/Sojourn/Soldier/Widow/Ashe
Is it really a wonder why Sigma is so good in top ranks? He is the only tank that can counter hitscans without having to run a full dedicated dive comp.