The one thing it's missing for me to ditch Firefox Sync for KDE Connect, is a way for me to store sent tabs while I'm not near my computer or when it's off.
Firefox sync is really neat as well, though. I like not needing to configure Firefox for each system I use when I can have everything sync'ed. I know Mozilla ain't the greatest but they're fine enough for my daily usage.
If I need full privacy I can go Librewolf or Tor browser.