In the past few months I have done at least the following transactions on points.com involving Delta: DL to CO, DL to VX.

Occasionally a trade of mine will be "under review" for a week or two and will then usually clear, but sometimes get cancelled. I suspect that when a trade goes "under reivew" the problem is more commonly with whoever posted the trade rather than the person accepting a posted trade, since the person accepting the trade can presumably be vetted immediately for being within trading limits, credit card authorization, etc., but some time has elapsed since the person posting the trade was similarly vetted. However, I have seen trades that I have posted go "under review", and I have never learned the reason why (and I would not expect to be told if the problems are with the other party).