I don't think that a combo system is intrinsically good or bad. There is no such thing as a "complex" rotation, because any fixed rotation is going to become auto-pilot when you do it for long enough. The one downside to having a lot of random procs is that it creates a lot of variability in damage output, and it can be frustrating to play if you don't get your procs. MCH was historically very proc based.

At the end of the day, it's really the fights themselves that determine how engaged you are. For tanks and melee, that means "how do we maintain uptime while doing mechanics?" which comes entirely down to positioning and movement. What does that mean? Bosses that follow a fixed movement pattern are boring. Bosses that the tanks are able to consistently move to a fixed set of safe-spots are marginally more interesting. Bosses that the tanks are forced to make decisions on the fly about the safest route and coordinate with the rest of their team to optimise uptime are the most interesting.

It's a puzzle. Also, it should be one which does have a solution. Mechanics where you deliberately force a tank off a boss to go to afk in some other corner are incredibly irritating, until we find ways to cheese them. The ideal melee mechanic is one that you can play with 100% uptime, but forces you to really be clever on the fly and really be spot on with your sense of timings to do so.