How good is Nextcloud's file syncing? Is it usable for gigabyte-size libraries? How gracefully does it handle conflicts?

I honestly don't trust it for conflict resolution. I keep data that is edited in multiple places, potentially simultaneously off of it and use git instead. I've had one minor incident with corruption in the last ~year, although I already can't remember the exact circumstances. Large files are no problem at all, I've even streamed movies off of it.