SyncToy may be fine for bidirectional sync, but the 'echo' mode is seriously flawed in my opinion. It doesn't do what you expect. I assumed the echo mode would work like this. You've specified a one directional copy from left to right. So you're saying the left side is the model for what you want. When you run the program you expect it to do everything necessary to the right side to make it exactly the same as the left. This would includes overwriting files in the direction left to right and deleting anything that shouldn't be present on the right. In other words the overall effect should be the same as if you had deleted everything on the right and copied it afresh from the left. The documentation seems to support this.
From my experiments the program doesn't work like that at all. There are two gotchas.
(1) It will only copy files from left to right if the left version is newer. If the right version is newer it doesn't copy that file and fails to warn you.
(2) It will not delete anything from the right unless it had previously been present on the left and then deleted from there. So a file or folder that you added directly to the right remains there forever. Again, it fails to warn you.
So it's not producing a perfect mirror at all. In fact, what it produces seems of dubious value. It's certainly no use as a backup, since you may have different versions of files on the left and right sides. Microsoft allude to this (but don't fully explain it) in the FAQ mentioned by Gnoupi.
They suggest a workaround by previewing what would happen if you synced the other way and then manually altering the files, but this is far too much work. Joel in Gö is right - it's the wrong tool for this job.