I was told by the guys a wisp-router that the rb14 should not be used for the SR2/5 cards. They told me that the people at Ubiquiti said that you should not use the SR2/5 in any PCI to miniPCI adaptor at all, because it will burn the cards out.
Has anyone heard of such a thing?
pci - mpci adapters is fairly extreme hardware when compared to normal pci adapters. So are sr5 cards. The two probably could get in a hissy fit about all sorts of things! We all know how finickey most of this crap is but I think someone is probably exaggurating the effect. Make sure you have hot spares and that someone over there likes you enough to take a rma on fried cards! 
You can’t use 4 high power cards in RB14, it would1 damage your equipment.
I suppose, you may use two high power wireless cards in RB14,
but they have to in correct place
(insert them one side to J10 and J11,
or to J12, J13)
I am using 2 SR5s in an RB14 without issue (as far as I can tell). MT does have a modified version of the RB14 available only through them allowing you to use 4 SR5s.
I us etwo SR5/2 in the rb14 too. But it was odd that I was told that.