Sync is charging for an ad free experience and/or extra features in order to fund the app’s upkeep and because the app is the dev’s source of income. People are torn about this because other apps are free, but generally aren’t as polished and don’t offer as many features, plus the sync dev has a good reputation and a long history of appreciated work. They also charged for similar stuff on their reddit app, but it was significantly cheaper. The prices seem experimental based on community feedback, so they may be subject to change, but some people see any pricing as a bad thing no matter what, while other people are in favour of supporting the dev even at the current prices. So far personally I like sync the best out of every app I’ve tried, and it is very similar to their reddit app which I used exclusively, but I also haven’t seen any ads like other people say they’ve seen, no idea why, so my own experience is coloured by that. That’s the issue regarding the free part at least. In terms of open source, it was made by one dev iirc, or at least has a small dev team, so it isn’t open source. But closed source isn’t automatically bad, people have just sorta gotten a bit open-source obsessed on lemmy and some people act like anything that isn’t open source is evil. The main issue is the pricing stuff with most people though.

same and idk what FOSS is

Sync is paid