As a current Thunder and previous Syncfor Reddit user, I can understand Sync's charm.
It's a lot polished, the most polished Lemmy client right now. There's absolutely no lag with weird config (e.g. link preview for Thunder), hell lot of customisation, ability to backup and restore settings, ability to parse virtually any links within Sync and even provide download option, etc.
Sunc benefits from using same old codebase with backend tweaking to Lemmy.
Thunder will ultimately catch up to that, but saying Thunder is current state is perfect and matches up with Sync in features is big hyperbole.