Couldn't agree more. He's bad with the clock and he fails my biggest test for any manager/coach: is the whole more than the sum of the parts? Mike Zimmer always felt like he had to be handed exactly what he needed to be successful and never coached anyone to be more than that. He didn't take a bunch of grinders, use what they do well, and fit his defense to them. His whole always felt LESS than the sum of it's parts. Even when the defense were good to great, did they ever feel intimidating? Did you ever feel like the Vikings were up by 3, needed a cold-blooded defensive stop, and you just knew they'd come up, kick ass, and take that game? I never did.
Zimmer hired bad assistants that couldn't coach offense and forced them to coach his way. He never found a coach who could help his offensive line or his corners progress. Or his quarterbacks. He just needed to go.
Spielman's biggest sin was allowing Zimmer so much freedom and deference to make all those mistakes. Chief of which was letting him hire family.