The one constant throughout my career was transience. I worked with so many different Art Depts. I noticed after a while a common problem. The Art Directors were often questionable. Questionable in the way that they weren't doing a good job managing their crew.
It also looked like a tough job. Stuck in the office, no creativity, blame, late nights, all-nighters...
I was educated about the "Peter Principle" early on in my career. I think it may be part of the reason.
But the apparent general Art Dept problem hinted of more.
Most people enter the Art Dept because they are Artists. The Art Director on a large Production has a non-creative role. Most think that the logical path to breaking into Production Design is via Art Directing. This isn't true because of the Peter Principle. Combine the two scenarios and you've got a bunch of wanna-be creative Designer types playing the role of Art Director. (A better route is Art Assistant or Set Dec)
Good Art Directors don't want to be the Designer (hence rare), they want to make it happen seamlessly. They are good at their job. Therefore their heart and soul is into their position. Good Art Directors need to be mature enough to promote the lower talent around them without feeling threatened. Good Art Directors do this easily because they are secure that they play a valuable non-creative management role. They are indispensable.
If you're playing Art Director and you know you suck at it, don't do it. It will do your career more harm than good. Art Directors are a different charachter/personality than most Artists/Designers. Good Production Designers know this, hire accordingly and are those Designers that end up holding onto their Art Directors for years.
You're as good as your team.