The caveats attached to these deals are all time-based and surprisingly complicated. For instance, that Assassin's Creed 4 discount is only available to UK PlayStation 3 owners on the PlayStation Network and draws to a close on December 21. The Far Cry 3 deal however, which can be found on Ubisoft's online store, runs until January 5 and includes a swath of other games such as Splinter Cell: Blacklist, and even a few discounts on non-Ubisoft games like Fallout 3. A cursory glance at the Ubisoft store shows The Walking Dead's PC incarnation at $6.25 and the Fallout: New Vegas Ultimate Edition at a mere $5.
If sorting through the reams of discounts seems like too much work, here's a trick that might help: While you can find all of Ubisoft's discounts by navigating its site, those who visit the Winter Sale page are able to pull up lists of games based on the size of their discounts.
Edit: The text of this article has been altered to clarify the terms of the Assassin's Creed 4 discount.