Speed, quality, safety, and seed status are the main benefits IMO. The downsides are you have to keep a good ratio or at least not hit and run.

Back when I used public sites I remember most torrents being slow, in private sites many people use a seed box so even if there are only a couple seeds it's usually still blazing fast. Since uploaders in private sites have some reputation to upkeep, their releases will usually be quality. I also feel completely safe downloading something with only a couple seeds on private sites, but on public sites I worry if I'm downloading a virus if there are no comments and very few seeds.

The private sites are also usually not big enough for anyone to care about, so the chances of them being taken down or targeted are minimal.

I have also not gotten one ISP warning since moving over to private sites years ago, and that's even with not using a VPN