We're using it in Quassel, and as soon as you go over ~3-4 parallel write/read threads, it starts locking up completely, sometimes taking 30 seconds for simple queries that should really take milliseconds.

The big issue is that sqlite does full db locking for any operation, so during any write you can't just easily read at all.

This can be fixed with WAL mode, but WAL mode is broken in uts early versions, and new versions of sqlite aren't in all disteos yet, despite being out for almost a decade. And even WAL mode gets abysmal performance.

It really can (LXD cloud setup from personal experience), the problem is that if you don't serialise your writes then yeah, fun times to be had. There are compromises for all databases. People just like telling others their opinion as fact, and how wrong everybody is apart from themselves of course.