The agenda seems to be that a lot of folks insisted Sheahan absolutely sucks. So if he starts to actually contribute and play well, that means them writing him off right away makes them look a little foolish.
It's really weird, you see posts like "Sheahan sucks defensively" or "Sheahan is horrible on faceoffs" and they're all just completely wrong. Sheahan so far in Pittsburgh has a 1.6 GA/60, 52.02 CA/60 (that is a very good CA/60, especially for someone who's starting in the defensive zone as much as Sheahan has) and has won 53.5% of his faceoffs while getting ridiculously heavy defensive zone starts (64% defensive zone starts) and nearly 3 minutes a night on the PK. He has been great defensively so far, but people still say stuff like "he sucks on faceoffs" and "he's horrible on the PK".
The issue Sheahan has had so far in Pittsburgh is that they haven't generated enough shots on net at ES with the 3rd line, but that has been different since Sheary-Sheahan-Hornqvist became a line. It is just a small sample size, but Sheahan's CF% is at 62.9% and his CF/60 is at 59.1 while getting ridiculous defensive zone starts (71.0% defensive zone starts). It remains to be seen whether that line can continue being effective, but it has worked really well both offensively and defensively (only 34.8 CA/60, which is downright insane) in the 3 games it has been together.
My only complaint with Sheahan so far this year is that he needs to shoot the puck more, that's really it. Sheahan has 13 shots and 26 shot attempts in 12 games with the Penguins, which just isn't nearly enough. You don't need to be Kessel out there, but he needs to just start throwing it on net more. I've noticed he's tentative with his shot, which is to be expected when he has scored as few goals as he has in the last year. In his best goalscoring season, he had 220 shot attempts in 79 games. He needs to be closer to than, not where he's at now.