I'm using Mojave 10.14.6, and my Chrome is Version 77.0.3865.120. I've downloaded several groups by this point, and it's gone well. My main problem has been running into Yahoo's rate limiter pretty quickly, especially for big groups, I've been using VPNs to get around that.

One thing I have noticed is that if messages have been deleted from the group, the totals you will download are less than the number of messages. This is something nobody can do anything about, but it was a bit confusing to me intitally.