As it currently stands I'd honestly want to say that Mirkwood Outposts are more useful to Lothlorien than their actual castles. Other than unit upgrades it effectively does everything the castle can do but better. It produces ranged cavalry, elite pikemen, elite swordsmen, elite archers, all of which should (according to the late-game meta) be replacing your units rather than supporting them. It produces two of the better heroes of Lothlorien, and can have anywhere between 15000 and 20000 health for roughly 3000 resources. The only other faction that has such a reliance on Outposts is Mordor, and they still have a number of options without Outposts.
This reliance has two primary concerns. On maps with Outposts they have a significant advantage if they can get one early (as they also produce resources and are at base cost relatively cheap compared to other faction Outposts), and without Outposts their difficulty is increased significantly. The balance of a faction shouldn't rely on the map being played (of course map balance is always going to be an issue but this is a reliance on Outposts rather than map design).
I've no idea how such a problem could be fixed considering how ingrained it is into the entire stratagem of Lothlorien, however the over reliance still seems a problem to me.