Simmons is DEFINITELY a diva. At ESPN he got near total editorial freedom, an expensive vanity site, executive produced documentaries, and a very large paycheck. Every columnist in the country would kill to get what he had. And yet he complained constantly about how he didn't get "support." On ep 1 of his new pod he was complaining about how ESPN didn't let him run enough ads, kind of weird that he was so mad about not getting a chance to plug Me Undies and Squarespace. He also has a thin skin and lashes out at any criticism.
That said he's undeniably a very good boss. The people who worked for him want to do so, and even the guys who aren't coming back (Lowe, Browne, Morris) keep in touch.
I guess it comes down to whether you think he's just an impetuous diva, or running into corporate dumb-dumbs constantly. Simmons does dumb stuff himself all the time, but given that it's ESPN we're talking about I'm more inclined to believe the latter. At Grantland I think his complaints about not getting any promotion from ESPN itself were certainly merited. Lowe and others are now splashed all over the front page all the time, but that definitely wasn't the case for Grantland. Simmons can do childish things, but it's hard for me to feel any empathy for the clowns running ESPN.
I think Simmons will continue to succeed though because he draws his fans wherever he goes, and couples that with actually hiring really talented people to compensate for his many blind spots and weaknesses (so it's not like a Rick Reilly situation). The one blind spot I wish he'd recognize though is that he's not very good on camera.