What I do is just create a player object when they join that that is set to autosave every min (as well as leaving) if one of their stats have changed. As for ensuring their data is not lost the object will hang around the server (after the player leaves) and retry saving their data every 60 seconds until success or something terrible happens.
I think its really convenient cause all I have to do push the data to the object and not have to think about it all.