The spy has to give up his only means of long range combat for the suit. Besides that, the number one frustration for spies is that they hide and are uncovered by an trigger happy pyro that just sprays down the area out of sheer habit. There's no controlling that sort of problem other than making an flame-retardent option. Besides, there's plenty of ways to spy-check already.
why is that pyro habit you should ask. It's because that's really the only way to sweep a base for spies.
The FaN I didn't see as an effective spy detector, because you couldn't look for cloaked spies even if you wanted to
I've been wanting some sort of area affect emp, or at least let scouts unlcloak/disguise spies when they touch them, like back in TFC
the change to the blutsauger so it doesn't trigger the health effect was salt in the wound.
I dislike spies, it seems that even when I know there is one near, and I spin around looking for him... I still get backstabbed. Fire has been our only reliable method for finding a spy and keeping him from slipping away.
I used to play the game Planetside, that had cloakers. Maybe their solution could work. Give one of the classes some glasses or goggle unlocks, you switch to them and press mouse 1, and it will put them on, then you can go back to your usual weapon. When equipped, can see through stealth and disguises, but you can only see about 15 feet, outside of that it looks like dense fog.