It's hard to quantify the different things that can help to build somebody's reputation as a "successful coach". It might be about building a player's understanding of X's and O's on the court - what to do and when to do it. It might be about promoting a physical makeover, change of diet, and/or embracing new training techniques for a player to survive long, grueling matches. And some of that success might be derived from the coach's ability to coax some players into a more focused and confident mindset. Even if the coach is full of it (dare I say a "confidence man"?), he or she might have a magic touch with cooking up greater confidence among clients on match day.
I've only caught a few snippets of PM's coaching ideas, but he gives me the impression that he's a bit of a tennis whisperer. He seems to work toward a mental calmness and focus in his players. Decent coaches generally seem to have a terrific way of communicating in simple terms, "You can do this and here's how." Even if he's not an all-time master technician, he seems to have a green thumb in terms of cultivating belief in the players that work with him. Maybe it's the accent...
Agree with your general outlook. I do think that Serena and Patrick are a bit of a different sort of case - they were both established grown-ups ($$$!) when they got together and I doubt that either one had so much power or leverage over the other that there was potential for abuse, etc.
But when let's say a young lady who is a relative newcomer to the tour hits the road with a coach, I think that's where there's plenty of potential for unprofessional conduct to happen. The player in that case is much more at the mercy of the coach.