From what impression I've seen, all this turmoil really can be just pinned down on the upper decision makers and their expectations and methods. Their lofty expectations that Sunderland can and should be challenging for promotion, while at the same time doing little to bring in any experience, having no managerial stability, and to hope to rely upon very young squad is entirely unrealistic combination.
There are some good players at Sunderland. There is some good talent that will be better next year or after, they need to develop. It's not the squad I'd expect to actually win promotion this year. They do themselves absolutely no favors with this managerial merry go round, seeming to hope to desperately squeeze into the playoffs to somehow fluke a promotion. Too much shortssightedness in the board room.
Sunderland can be as capable, maybe even more capable, as any club in this division to achieve promotion. Not that I really want THEM to be promoted, their size and history doesn't give them some sort of "right" to the top division. Neither does ours. It's merit based and good planning based. I don't wish them any specific ill will, don't wish them any specific good luck either. Rambles over.
But their current approach is highly doubtful to achieve anything but close-but-not-quite.