I am running a Midnight campaign and being a playtester of Arcana Unearthed, I found that quite a bit of AU fits nicely with Midnight.
AU is released on July 25.
Here is what I am using from AU for Midnight (and so far, no problems)
Classes
Akashic (as is)
Champions (as is)
Greenbond (cut the spell slot section and use spell points)
Mageblade (cut the spell slot section and use spell points)
Oathsworn (as is)
Runethane ((cut the spell slot section and use spell points)
Totem Warrior (as is)
Witch (cut the spell slot section and use spell points)
I am using the AU skill and feats without any problem (more correctly, noone has taken any skills or feats yet that are problem, so I can't vouch for 'all')
I am using the AU death's door rules and hero point.
I am using the AU spell system, modified to Midnight. So spells can be heightened, diminished, laden, and have spell templates.
I haven't encountered any problems so far. And to answer the probable question in your mind - No, the inclusion of this material has not changed the flavor of Midnight at all.
My two coppers....