Obviously the big things were the advertised Ultra features which were still missing. It sounds like those are being worked on now, but it was disappointing to be paying for Ultra for half a year without those.
The other concern, though was that changes in Lemmy itself left Sync either broken or missing features. It was in fairly minor ways (although "scaled" sorting was definitely missed), but I worry that in the future something will change that breaks it in a more drastic way, and if we're getting five-month gaps in the development of Sync, that's a long time for it to be broken.
Don't get me wrong, I think Sync is the best Lemmy app out there, and I'd rather use it than anything else, but the big justification behind the price increase had been that it was going to receive more constant development, and so far that doesn't seem to have been the case.