Can't you already see buffs in clouds?
I don't think so...
And I am not thinking of this suggestion only because it's used so much, that was just a less significant thought I had.
My suggestion gets more significant when it comes to you trying to stay hidden. Sneaking up (if executed right) already gives you the advantage of getting the first hit and usually leads to the killing blow. But if buff were to be visible inside clouds you would have to give up superior engine/balloon/hull power to be able to ride a cloud in secret. Even though you can get into a secure position behind the enemy and then buff everything...
It's not even that much either, since you would not see it THROUGH clouds.
Also, as a side note, there are already many teams that give up their gunner, just to have an engineer (Mallet/Spanner/Buffkit) that can buff his own guns and enables more repair power than a gunner does (but that's another story).
Another thing is that aside from gun buffs, buffs usually hold quite a while and a well timed/placed buff can still hold after a short engagement.
I don't think you really have too many downsides...
