The key difference between manifest V2.0 and V3.0 with regards to uBO is in V2, the extension has direct access to the browser’s process in making web requests and can make direct changes to those requests. V3 instead requires the extension to declare a list of urls and the browser will act on the extension’s behalf. This is a very simplified explanation that isn’t in any meaningful depth and misses a ton of nuance.
The outcome though is V3 makes it significantly more difficult for uBO to achieve its goals for its users. It is a downright and explicit downgrade, and when Chrome fully moves to Manifest V3.0, uBO’s ability to serve its core functions will likely be diminished.
As another example, the Path of Exile community moved off onto their own community-run wiki domain, but the Fandom variant (which is woefully out of date) continues to be one of the top results when searching for a PoE wiki page.
In some regards that’s inevitable, but it clearly shows what Fandom’s priorities are.