This post is from the WWDC26 Audio Q&A.
AUv3 was introduced back when iOS 9 came out, and it only had very minor updates.
In a way, it didn't seem important enough for the desktop world, as they continue to use AUv2, and only iOS musicians/developers embraced AUv3. Even Logic Pro doesn't feel like it fully embraces it. It is almost like there was no good enough reason for the desktop world to switch over.
Also, because of this, I think many bugs remain. One of the bugs I keep hitting, I think, is an AUv2 to AUv3 API mapping issue. Where AUv3 uses KVO for certain properties, and somehow doesn't work well if the host uses the C API.
Here's one example: https://developer.apple.com/forums/thread/828549