I am using postgresql now. That is not a silver bullet for anything though. I switched to postgresql early on (when I first started using matrix I think synapse only supported sqlite?) and I've had less disk space & memory trouble using sqlite than I have postgresql.
Of course I was using sqlite when there weren't nearly as many users as there are today or when I had federation disabled.
Of course I was using sqlite when there weren't nearly as many users as there are today or when I had federation disabled.