The 50% thing has been beaten to death so never mind that. What i have doubts about is the following:

Can i have some units deathwing assault in turn 1 and some in turn 2, or must all units deathwing assaulting arrive in the same turn?

The way i see it, the sentence "All units making the deathwing assault automatically arrive at the start of the chosen turn", could easily be refering to the turn chosen for each individual unit. At least that how i instinctively read it the first time around. Now im not sure...what do you guys think ?

A RAI point is the fact that they bothered to give Belial a homer. That would make a lot LESS sense if all deathwing assaulting units are supposed to arrive in the same turn.