I've been using FreeFileSync for a couple of years now, first on Windows 8.1 and now Windows 10, both 64 bit.
I've used a custom install to eliminate some of the bloat, and so far (knock on wood!!) I've never had any issues.
The program works great!!
I've been using v7.4 since it came out; no plans to change to a newer version anytime soon.
I also use MS SyncToy, which seems to run fine on my (now) Windows 10 laptop, also 64 bit.
Randy