Imo a big issue that has plagued timeline from the beginning and sadly still plagues it is:
It's still fairly unstable, leading to crashes from time to time.
While I see that the timeline crashes get fixed when reported as a bug, it sadly seems that new crashes crop-up almost every update and it also seems to to get harder and harder to reproduce them.
I understand that the timeline is a fairly complex features and I love all the new updates and qol changed, but it is a really bad sign that it still is this unstable after being out for a few years by now.
This leads to the case where a lot of community members are actively avoiding using the timeline.
Because of that reputation it's even more important that the timeline is 100% stable. To overcome the fear of crashes and reinsate trust in the stability, the timelines robustness needs to be proven double.
The timelines stability needs to be looked at thoroughly, tested and refactored to bring it to a robust, stable state imho.