In the FAQ section, the answer to “Can I use Firefox Sync with LibreWolf? Is it safe to do so?” is that “There aren’t any major privacy or security downsides in doing so, as Firefox Sync encrypts your data locally before transmitting it to the server”.
So why does LibreWolf disable it by default?
yup, the idea is to ship a de-cluttered and straightforward browser. however we figured there was no reason to completely block users from using Sync, so we made it easier to enable and added the faq entry for those who like the convenience.
it’s just another option, like xbrowsersync and manual backups.