When submitting edits (manual edits or API calls using WBI), I frequently get response 502. When retrying, the edit normally gets through, although I have had to retry manual edits up to six times. WBI, when recieving a 502, sleeps for 60 seconds; after that, the edit always gets through. I am editing https://wikibase.inguma.eus and https://lexbib.elex.is, two of the wikibases migrated to wikibase.cloud in Batch A.
When I have two processes run on the same wikibase at the same time, 502 seems to happen much more often, although I also get it quite frequently when having a single bot running. When having a (WBI-based) bot writing data for a while, parallel (manual) edits will get 502.
I also have had WBI item write actions hanging, until I made a parallel edit; then, the hanging process woke up again. I use [[ https://github.com/LeMyst/WikibaseIntegrator/tree/rewrite-wbi | WBI version 0.12 ]].
When using QuickStatements, I have not experienced this problem.