As someone who often plays on Ultros at US night / EU morning / JP primetime, yeah the server is heaps quiet at that time: no party finders (or 2, tops), pretty empty towns, maybe 5-10 GAF FC people online either (semi) AFK or minding their own business... Duty Finder queues will vary a lot depending on the content in question - from 'reasonable' (0-30 min as DPS) to 'not worth it' (60 min or more).
Also, most maintenance is timed to end roughly 9am GMT which will not help.
Weekends are a bit more active than weekdays, though, and new seasonal events can attract a crowd even at those hours.
I realise I'm not selling this timeslot at all, haha. I swear it used to be less dead earlier in the game's life e.g. before EU servers existed.
Oh and yeah, you can forget about open world content (world boss FATEs, A- or S- rank hunts). Those spawns are pretty much wasted on those hours.
All that being said though, its an alright server. Just not optimal at those times.