The sites I had WPMU installed on were getting timeout issues.
A little information in case it helps:
Host: Siteground (several different accounts)
Themes with the issues: Divi, Ocean WP + Elementor, Customizr
Issue - frequent 504 timeout errors (resolved after removing WPMU plugins, sites not using WPMU on the same hosting plan did not experience 504 timeouts).
I believe the issue was with snapshot or the WPMU hub dashboard. Those were the only two things active across all the sites that had issues.
I have a demo site that has had this issue that I could send you if that's helpful… (It's a blank site I use as a starting point sometimes with clients).