Actually It matters.
Even you either way have to play through Uplay if you don't add this game to your steam account than steamgifts database will think you didn't activate the game you have won.
And it will cause some troubles in the future.
So please be careful.

It does matter. This is steamgifts and not uplaygifts. The key has to be a steam key or no checks can be made. I'm aware it has to open through uplay but the winner must own it on steam in the end.

It does matter. If the winner is given anything other than a Steam gift or Steam key for the promised game, he or she is required to mark the gift as "not received" and the creator of the giveaway receives no value for it.

It super important to me, Im regret buying games from Uplay, because you need to buy DLC directly from Uplay and it expensive, if it steam, you will get cheaper DLC from time to time.

Downloading is speedy via steam. at least for me. cause steam uses cached download server

it matters because of the rule's site

Strangely, there are several Ubisoft games (not just ones published by Ubisoft, or older games, but even newer ones developed by them) that activate through Steam exclusively. The games'll use Uplay the same as any Uplay-linked Steam game, but they'll activate and play through Steam.
Example: http://store.ubi.com/us/atomega-tm-/59bfdc2cef3aa581481019f0.html

As you noted, it may just occur like that due to Ubisoft wanting to exclusively make use of Steamworks for multiplayer, so as to keep a consistent multiplayer connection between both platforms. Then again, forcing their users to utilize Steam for a game is rather counter-intuitive, and given that their Steam games utilize uplay to begin with, it's a bit uncertain why they don't default to using it as the basis for multiplayer connections for both platforms.

What's really weird, I suppose, is when the Ubisoft Store sells steam-exclusive games not developed or published by Ubisoft, like Injustice: Gods Among Us. Then again, as I understand it, Ubisoft has a solid working relationship with Digital Dimension, who has done the film sequences for both Injustice and some of the AC games, so that may be the connecting element driving those kinds of inclusions.

This is exactly why I asked...

I'm no new, this guy have create this Giveaway and other of AC Unity in his first hour in this website, it's suspicious, very suspiscious, that's all, hope not a scammer but seems like one

Thanks for support

I'm sorry, since I was never faced with something like this, I didn't know I shouldn't say that.

Ahaha :D