There is no loss of events and flows.
Note: When you perform this function, all changes made to the settings after the backup was created are lost
I didn't try it myself but I assume it depends on the type of ESM backup you've made. A standard ESM backup will include the configurations only and optional the events | flow | system logs. In case of a restore I assume it will wipe the database and restore the one from the backup.
Let us know what kind of backup you want to restore and how it went :-)