GEO currently suffers from having a
very niche role of being;
a unique debuffer (which consequently gives alliance-wide buffs) against a static target. It performs in this role really well.
Any time you stray away from this very specific niche role, whether its by adding additional targets that are spread out (ex: plasm farm), or by lowering the party member total and low-maning content (ex: abyssea content), GEO sort of gets... lost... out of place. Sure they can function, but there are other jobs who could take their place at that point and perform more efficiently.
Although the bright side to being wedged in this niche is that GEO's will always be in high-demand for any 'alliance vs. 1 super-target' content (which makes up the trend for current and upcoming end-game content)
Maybe I'm wrong and some more veteran GEO's could shed some light on their experiences, as personally I've barely gotten to play mine. This is just what I've noticed from playing with other GEO's.