You just won’t find as many compatible external plugins compared to AE.
Wha? There are
endless plugins and templates for Motion. In fact, I bet you there are
far more since they are much easier to make. Just look at FX Factory or motionVFX for example. Nuff said. Motion's integration into Final Cut is exponentially deeper than that of After Effects in Premiere.
The plugins for AE are simply
different, usually for super-specialty tasks, and not all of them are available for both, but many are. So you need to know what it is
you want and need. Called "horses for courses." If you're using something just because it's
popular, you're doing it wrong.
I always say that Motion can do 90% of what AE can do for 100% of users. And almost all of it in real-time, unlike AE. The number of people that need the last 10% is minuscule. And if you don't know what those things are, then you don't need them.
I use Motion to make titles, effects, and generators for Final Cut Pro, and whatever graphics animations I need. Using After Effects in the context of Final Cut is shooting yourself in the foot. Just like using Motion with Premiere.