People probably won't look back at it like this because football has trended towards "winning = brilliance" but if you analyse the performances of this Real Madrid side, they've been very poor for 2 of his 3 seasons in charge, and yet they've won the CL in one season, his first, and probably will in this season as well.
What they are is a testament to the ability of world class players to take responsibility on the big stage and produce winning displays on their own.
Zidane's tactics are very basic and rarely effective, but they have such a familiar side full of world class players that they can just keep themselves in games until Ronaldo, Bale, Isco or Assensio win the it for them.
In the league week after week where Ronaldo and co. aren't quite so desperate to steal the show, they fall short time after time, because the big tactical flaws, like their entire left hand side, get exposed over time.
Zidane keeps the ego's in check because he's the biggest name in the room and they can't argue back. He has more power than Perez because of the fans, so he's the perfect glue to keep them together but he's no better than average as a coach, I would be amazed if he succeeds anywhere else.