Came from using Sync for probably close to a decade on Reddit, so I'm definitely a creature of habit switching back for it on Lemmy. That being said, I tried ALL the apps looking for a solid experience and nothing delivers like Sync (opinion). All of them were missing something that completed my experience, was that experience coloured by my time with Sync on Reddit, sure. For all the options Sync gives me I'm willing to pay the dev half the cost of a steak dinner. I've already spent 100x longer on Sync for Lemmy than it would take me to eat said steak.
As for FOSS, let's just say it's not the be all / end all. Look at all the options that Sync brought to the table in a month that Lemmy hasn't added in the whole of its development.
Personally I feel like most of the folks complaining about Ads in Sync are just complaining to complain. The FOSS crowd is also the most likely to use ad blocking, filtered dns providers, pi-hole, adguard etc and if that's the case they won't even see a single Ad.