I have been using the Echo function in SyncToy 2.1 to backup the data on my laptop. From the description it says that this function should make the folder contents on back drive identical to the folder on my laptop. On closer inspection, it hasn't
deleted or added files that I have added/deleted on my laptop. The only thing it seems to do is update the files that I have modify. I don't have any filters turned ON and tried creating a new pair which also didn't delete the additional files
on my back-up drive.
I've experienced those problems with Echo with both SyncToy 2.0 and 2.1. Echo has never worked properly for me, even when starting with a completely new external drive with nothing on it.
IIRC, the bugs with the Echo feature were supposed to be specifically addressed in SyncToy v2.1, but in my experience, nothing changed from v2.0 to v2.1.