Regard to shield tanks, every one of them is designed with certian limitations.
Winstons is....well, it used to be the weakest of all shields, but it covered the greatest area. And, attached to Winston's jetpack, it allowed itself to be placed anywhere Winston decided to dive. In contrast, Orisa can only place her shield in front of her. She can shoot it out over a long distance, I suppose, but that's very rarely optimal, as it's shape is meant to encompass her. But once that shield is placed, Orisa is committed to staying in that spot for atleast 10 seconds. If the enemy flanks, she can't reposition the shield and, lacking mobility, all she can do is hope to survive the battle (which she might as her other abilities are handly in keeping her alive). And with Rein, his shield has the greatest hitpoints, but it's deployable at will, but perpetually attached to Rein himself, who has abiltiies that can help him in a brawl but do little over a distance.
So...Sigma
His Shield is attached to his person, like Reins, but is deployable to an area, like Orisa's. But his commitment with it is much closer to Rein's since he can recall and redeploy it with basically just a one second cooldown. His abilities that pair with it are like super charged versions of Rein adn Orisa's. Where Rein's brawler capabilities were limited by his range, Sigma has extreme midrange damage and even a stun on cooldown. Where Orisa has fortify to shield her when her shield breaks, Sigma has an absorption ability that gives him more shields.
So, it's honestly like someone took Rein's and Orisa's shield, then paired him with abilities that scrub away any limitation that shield leaves him.
Like Baptiste, Sigma feels like he has one ability too many. If he is meant to be an attack tank, one that can brawl with dps like Roadhog or D.Va, then he shouldn't have Kinetic Grasp. He should have his shield and otherwise rely on his offensive abilities. Or, he should be a defender tank, something like a more mobile Orisa, and just be super hard to actually do damage to and lack that Accretion ability. But if he has both, then he basically has no weakness.