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Sadly not completely perfect.
Seems Stylish Sync has problems if your stylesheet collection is too big...
I submitted a bug report over at google code, so if you'd take a look I'd certainly appreciate it!
But there's nothing really to be done here, as explained in http://code.google.com/p/stylishsync/issues/detail?id=1#c1 .
You might try one of the suggestions there.
It's a limitation by Mozilla's sync servers.