I think Tomlin gets much respect for his two SB rings. One was with Ben the other as an assistant coach. Right or wrong when you win a Lombardi as a coach it elevates your status. In short, you kind of get the benefit of the doubt. Sometimes their coaching skills aren't inflated. I think Mike McCarthy is a good example of that.
Another factor is having a franchise QB. When a coach has an elite QB expectations are elevated pretty quickly. There is a different kind of pressure on the coach. I think Sean McDermott is a good example here.
With that being said, Tomlin doesn't have a QB. He should take a hit for drafting Pickett because he's not franchise. Not sure he's even serviceable. Nevertheless, he has his team in the playoffs and arguably played in the toughest division the league. I don't think many thought Pitt would make the playoffs.
IMHO, the Steelers have over achieved. No one is expecting them to beat the Bills. Truthfully, all the pressure is on Buffalo. I think the Packer and Cowboy game is a good example here.
Hopefully, the Bills can play well and beat them today.