Whether it's an actual problem or not I think people see taunt on a frame without innate DR and nope out of Styanax. He's also an amazing specter so if you want that aura all you have to do is drop him. There was also the nerf to Flyanax, assuming they never backpedaled that.
It's a bit weird for me to say 'Rhino is underused' since it's the frame everyone recommends to new players. But I think people forget about him when Rhino never stops being good.