Combos in general are things that can be used effectively or poorly. MNK's single target GCDs are an example of a good combo system, because it's not a 1-2-3... It's a 1-2-3 4-5-6, and sometimes it's a 1-5-3 or a 4-2-6 or other possible orders. There's fluidity there, and you need to keep aware of things like when you need to reapply Demonlish or when Twin Snakes needs refreshing. Its AoE combo, on the other hand, is not the worst offender thanks to Perfect Balance, but is really just a 1-2-3 > 1-2-3 > 1-2-3 situation, and that makes me ask the question of "what's the point?" That's three buttons--three spots on the hotbar that have no value in most combat scenarios since AoE is just reserved for trash mobs anyway. It's the same with MCH's combo. There's no branching there--no choices. What's the point of eating up so much space for something that is highly repetitive?

RDM's single target combos work nicely because it's also very fluid. You have You can start your 1-2 combo with Jolt, or Verstone, or Verfire, and any can go into either Verthunder of Veraero depending on your needs. It's in constant motion. Its melee burst combo is just three buttons that always go in the same 1-2-3 order though.

It's really just about how its handled, and preferably that it feels unique and different from other job combos as well.