Martin's issue was a shocking level of rigidity.
He did a really really good job getting them up with a squad that was 1. not PL quality when it went down and 2. had been stripped of its PL quality components.
But when did we get to a point where a manager being stubborn with their system became viewed as a benefit rather than a problem? This ties into the issues at City too, where Pep is showing a shocking unwillingness to adjust. Martin's stubbornness here boggled the mind. Southampton's very good possession game did them well to get them promoted (and his tactics are, I think, why they got promoted). But PL pressers are not Championship pressers! The lack of adaptability is best encapsulated in the 5-1 loss to Chelsea a few weeks ago, in which he threw his 4th choice keeper out to the hounds and asked him to play out of the back. Like, what the f*** is that!!!