I'm with you ^ i get "Uplay is unable to start your download. check that you have an active internet connection and sufficient space on your hard drive and try again" when trying to download and i have active internet and plenty of space.
I think they tried releasing to many games all at once AC:unity/Rouge/ Farcry 4etc.. all with in days of each other) and now we will have to wait a month or 2 for them to fix it and add more servers.