DD has experienced both. She prefers event coaches, for sure. At her current gym, they sort of do event coaches by level-ish. So, there's maybe four main coaches who'll coach beam, and two primarily do optionals and two primarily do compulsories... though there is some overlap. And there are also some part-time coaches who sorta don't have a niche per se, but might sometimes coach DD one day/week.
One coach does mostly bars/vault. A couple mostly do beam/floor, but one often will coach DD on beam, floor, and vault on different days of the week.
I'm having some trouble explaining this, but DD usually doesn't have the same coach for more than two events on the same day. This is an example, not by any means accurate:
Day 1: Coach A beam/strength, Coach B floor, Coach C bars
Day 2: Coach B beam, Coach C bars/vault, Coach A strength
Day 3: Coach A beam/floor, Coach B vault, Coach C bars
Day 4: Coach B beam, Coach A strength, Coach D floor, Coach E bars
Day 5: Coach F beam, Coach B floor, Coach C bars/vault
DD is 11, and is really liking having event coaches this way. She feels a bit less frustrated/annoyed - there's a little less "battle of wills". Say DD has two primary beam coaches and two that might occasionally sub... often hearing that "new" voice or perspective really helps my DD, even if it's technically a "lower level" assistant coach who's just subbing in on a given day. Sometimes, to me as a parent, it almost sometimes seems like some good cop/bad cop action when it comes to fears/head issues.
One girl in DD's group is moving on to level 7, and lost her flyaway the week before scoring out of level 5. It was nice to see coaches going into triage mode to try to help her - she had a variety of bar coaches throughout practices trying to help. She managed to compete it.
The down side of this is when a kid really, really does well under a given coach, and only gets them occasionally on that favorite event. Or like my DD, who prefers Coach A on beam (goes easier on DD), and coach B on floor (DD excelling here so she likes the more strict push here), and has to deal with both on both.