Sync is the odd man out because even though it's free to use, it's ad driven, and then there's a yearly subscription service for no ads.
Honestly, it's a good app with a good developer. If supporting good work is your thing, I see no reason to use it free or with subscription. It supports him, and if you don't like it...you always have FOSS apps which arent as polished but completely free.
FOSS apps are important because they'll always be the standard of "acceptable" and alternative apps MUST be better if they want you to give up your hard earned time and money for it.
That's free as in freedom not beer. So the big difference is that you and the community can look at the code and inspect it. Closed code you are just trusting the company behind it.
There are always ways to support Devs in Foss software as well. Just ask , everyone has somewhere to receive donations.
I for one A) want to know what I run on my devices. B) don't like to be spied on C) don't want to support the practice of doing so. And D) I have security and privacy standards. Even if you would trust a one dev closed source project , you are just one hack away from getting something nasty on the next update.