Thanks, narF, I will take a look at AnyType.
I am guessing that Joplin Cloud itself is perfectly stable, in some sense. It's probably the file-locking during synchronization that isn't well-implemented. (Sorry, easy for me to say, I'm sure this is extremely challenging.)
The most recent sync issues I encountered are posted here: "Some items cannot be synchronised. Press for more info." Now what?
In a nutshell: everything was fine until suddenly it wasn't. A few dozen conflicts were flagged but it wasn't clear to me what I should do about them. I "retried" them all on the mobile app and they went away. A few minutes later several hundred conflicts were flagged. I reasoned that things were in bad shape and decided to uninstall the mobile app and start over. It never was able to synch back up, even after multiple uninstalls/memory wipes/phone restarts.