As of 04/24/20: 10.15.4 - bug identified in macOS, problem when copying large files. Avoid updating or rollback.
Also see: support.apple.com/en-us/HT209000
Emphasized by @avidmarianna: read the README and WHAT'S NEW docs.
If you've upgraded macOS to 10.15.4, there is a known issue with local attached RAID storage. System will hang when running Media Composer or copying large files. Doesn't happen with earlier versions of macOS.
UME is not a replacement for AMA.
Again, the choice is between UME vs. QuickTime, which will differ from workflow-to-workflow.
Still the best way to work in Avid, transcode everything for max stability and performance. Which means, budget project time for transcoding.