It takes more than 1.5 hrs to finish and u get three items in the chest, you can do 2 or 3 dungeons in that time. I think they should increase amount of xp or rewards in the chest.
The XP is great. During cake week, with cake and ambrosia, I got about 500K XP per run. Each run took about 40 minutes. I ground two characters from pre-VR up to VR16 through nMA. There are better grind spots, of course, but nMA is, conveniently, your own private instance. The XP from group dungeons, OTOH, is quite low.