Older Games' DLC are useless if bought on Ubisoft Store
I recently bought the DLC for Ghost Recon Future Soldier, the thing is: you need to activate it with the proper key in-game even though it is already downloaded on your PC files DLC folder. I contacted support because the DLC was listed on my owned DLC but I couldn't see the key anywhere. Support told me that the store template didn't provide a key, which means that the DLC is going to be downloaded but you'll never be able to access it. The only solution was a refund.
Various older Ubisoft titles follow this system (in-game key activation apart from the download). DLC for Assassin's Creed Revelations follows the same mechanic. If you don't buy the DLC with the key elsewhere, the DLC key is nowhere to be found.
I think that the DLC key exists somewhere but UPLAY is just not able to show it... for example, the key that I bought for Assassins Creed DLC is not shown anywhere. Uplay has a "show key" option, but it only shows the base game key. Ubisoft, you should consider adding an option like right clicking on each item of "Owned DLC" and a "show key". If you don't do that all your DLC for older games on your store that require in-game activation are useless and people will have to buy those game in stores that show the keys for them.