I started to answer points 1 and 2 independently, until I realized that they are somewhat correlated.
When Sumlin was here, I thought he was as good as anyone in the country from a recruiting and Xs/Os standpoint. Offensive scheme was always cutting edge, and we always had really good talent.
However, I thought he was severely lacking in 2 crucial areas: leadership and people skills. Those deficiencies were somewhat hidden at Houston, as the "booster culture" here was nowhere near as intense as larger traditional powers, and he relied heavily on strong leaders among the players, guys like Case Keenum, Matt Nicholson, Chris Thompson, Marcus McGraw, etc.
I always thought that if he went to a place like TAMU, OU, Texas, etc., where glad-handing rich oil tycoons and schmoozing the higher-ups was a more integral part of the job, his personality deficiencies would start to become a problem, which apparently they have. I also don't think he is very good at leading a team of young men, and in turn developing leadership qualities in his players.
For those reasons, I always thought he might be more suited to an NFL gig eventually, where his somewhat abrasive personality might not be as big of an issue.