Can Uplay games be given away if they are purchased via Steam?

Not if they are U-Play only.

I mean when you buy an Ubisoft title on Steam it gives a Steam key but if you buy it on U-Play it just gives an U-Play key which you can not giveaway here.

If a uPlay game was purchased in the Steam store then it will be a Steam gift and redeemable on Steam. Which is fine for a giveaway. It just has uPlay as additional DRM.

If it was purchased elsewhere then it will likely be a key for uPlay only, which is no good for a giveaway. But there are exceptions.

If I can put the key on Steam and redeem the game on Steam, it's fine. If it's a Steam gift, it's fine.
Otherwise, it's not, no matter what's the DRM involved (Rockstart SC, Uplay etc.)

Voted "no", because you can't buy Uplay keys via Steam to begin with. You cant buy game keys on Steam period. You can only buy Steam gifts.

This key you receive must be activated in Uplay, because it link Steam and Uplay.
If activate that key in other Uplay acc (not yours) you cant play via Steam and the other guy cant play via Uplay.

Well some people write : "This is a Uplay key"- thus implying that you can only redeem it on Uplay. In those cases you can of course tell them that they can't give away the game here.
In all other cases it could be a steam gift. This normally requires registering with Uplay,too, but those are totally fine to give away here.

Let me say it like this: The giveaway-key must add the mentioned game to your steam library, when you enter it under "Activate a product on steam..." below the Games tab in your steam account.

edit: seems you already have your answer, cheers!

Right, I think I've got it now. Thanks everyone for clarifying, AND MAKING ME FEEL ALL STOOPID.