Mostly title. Craglorn can be soloed by a mid-powered v16 character (at least, I haven't tried anything else) - delves, quest bosses, basically the whole zone outside the Trials. But there are very, very annoying locks in the quests which require multiple characters present for no combat related reason.
This wouldn't have been nearly so much of an issue back when Craglorn was new and populated, but as of right now it is D.E.A.D. - and each DLC that gets released kills it further. Solo Craglorn is good fun, it's much more difficult even as a v16 fighting v10 mobs than any other solo content outside of vMSA - please let those of us who want to play it do so without these restrictions.