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?
Opt-in seems like a reasonable position for this optional feature. Not everyone uses it. I don’t. To turn the question on its head:
Is there a compelling reason to enable it by default?