Mystic adepts can spot astral threats; they just need to spend one of their power points on astral perception. Which is very advisable, since they need astral perception to erase their magical signature. Mystic adepts aren't really that OP, they just seem that way because they are front-loaded, power-wise, then progress at a slower rate. Astral projection may not be as appealing as having adept powers, but it does give a mage a unique role in legwork, tracking, and infiltration. Sure, a mysad can send spirits on astral tasks, but that is like a technomancer sending sprites out on Matrix tasks - cumbersome, limited, and inflexible, compared to doing it personally.
The biggest weakness of aspected mages is that spellcasting and summoning are both so powerful and versatile, and quite frankly, aspected mages don't get nearly enough in return for permanently giving one of these things up and not being able to astrally project. I mean, look at what they get for Priority: B. It's pitiful.