I'd not term Rob Ryan a "failure" either. But I do think personality counts for a lot - and while Rob seemed to be able to keep his mouth shut under Belichick and in other roles, as of late he's been emulating his brother.
That alone isn't necessarily enough to get him fired, but the question remains, is Rob Ryan as a DC getting everything he can out of his Defense - and that's certainly an area where personality enters the equation (with players and other coaches).
None of us know what has gone on behind the scenes but given the fact that he was let go tells us that the feeling in Dallas was that he wasn't doing enough or wasn't fitting in.
The fact that Dallas has been mediocre for 3 seasons, failing to make the playoffs is certainly enough of a reason to consider a change.
I know Rob Ryan remains confident in himself - boasting he'd be unemployed for all of 5 minutes. But last time I checked he's still unemployed, hasn't interviewed for any of the numerous vacancies, and it's been far more than 5 minutes.
So perhaps others look at the Ryan Family Foot in Mouth Disease (insert your own Rex Ryan joke here) and have decided to keep their distance.