Out of all the managers we've been seriously linked with, he's the only one with a Championship promotion under his belt.
He did struggle at Stoke, but they were a bit of a basket case at the time and they had some really random players that were on high wages. Sunderland was a decent promotion because they'd struggled to get out of that league for years and underperformed massively. One could argue he had a talented squad, but then the previous managers couldn't do anything with it so he obviously has something about him. His work at Hamilton often goes unmentioned, but they didn't even have the biggest budget in the Scottish Championship and went up through the playoffs, which is notoriously difficult to do in Scotland. They beat Hibs, who arguably had a much better side albeit in woeful form. In his first season in the SPL he won 12 out of 20, Hamilton only won 3 of the following 18 after he left.