OOUI has a whole list of features that can save a lot of headaches in the future for RevisionSlider, especially in terms of positioning of floating elements and popups.
- The OO.ui.PopupWidget for example already has mechanism to "correct" its placement for RTL/LTR and for screen sizes (by "clipping" to the screen width and height, see OO.ui.mixin.ClippableElement)
- Another point is that OOUI uses MediaWiki and Apex themes that are constantly updated by the UI Standardization group, and so its design fits the rest of modern MW elements, which could save a lot of time in designing elements and fixing and updating design in the future.
- OOUI also allows and is optimized for accessibility, which can save work on that aspect.