Aloha,
So the UU's were never introduced to either "buff," "re-vitalize" or due to any type of power creep occurring in the game.
They were introduced to specifically change how a ship can be utilized or employed in battle. Case in point the example I always use is the Republique's UU, it decreases her main battery reload time to barely over 19s while reducing her main battery gun range to 19km's, for some, this is not a buff. For others this makes her be played differently than they normally would play.
Many of the UU's simply were/are not effective due to their penalties. case in point Shimakaze, you pretty much HAVE TO stay on one course and use the ship as the launcher in order to line her torps up.
With the release of several new tier X's, we intend to add more UU's. Furthermore, when we talk about modifying the current ones, I am guessing some of those changes will be to make them, well useable and worth the purchase.
As soon as more information becomes available we will release it to you all.
Mahalo,
-Hapa