That’s simply untrue. Experimental Barrier is the best non-Ultimate barrier ability in the game, by a wide margin. It combines the best features of Reinhardt’s Barrier Field (huge HP pool) and Orisa’s Protective Barrier (ease of use, to the point of having nearly 100% uptime if managed intelligently), with none of their drawbacks (no cooldown; can be placed anywhere, including in midair; and doesn’t prevent Sigma from fighting). It’s not situationally better than other barrier abilities, it is better.
I would nerf Sigma by cutting Experimental Barrier’s HP in half, from 1500 to 750. That brings it in line with other barrier abilities; Sigma would still be able to protect himself and his teammates, but it would not longer be an “eff off” barrier.