Regarding new Synthsuit armour - around October 2014 there was a bug with multi stat modifying armour, making operatives wearing said armour unassignible to any transport and making them not appear during base defence mission (and I think you can`t "sell" them too).
I`ve lost quite a few nerve cells because of that, and I`m not sure if that bug is fixed.
Only time (to my knowledge) this problem surfaced was in Piratez mod, with late-game armours pumping operatives full of battle drugs and modifying a lot of stats, including STR. Singular STR bonus worked okay (Synthmuscle suit etc), but in conjunction with other stat bonuses you were guaranteed a no-fun time.
Only method of fixing it ingame I found is to remove said armour from operative (or equip some another one) and run any mission with different crew (even just making battlescape appearance and aborting should work, I think).
Posting this just in case this bug still persists, because FMP Synthsuit modifies a lot of stats, STR included, and someone might lose a favourite operative by just unassigning him from craft (or him getting a wound), never to assign again.