I did explain in another comment some general information about I2P. The one where I mention how it is a darknet but is much different than Tor.

The reason many more people don't use it yet is because it is hard to setup. That's pretty much it. Similar to lemmy or other things that exist it is just difficult to get people using it unless it is significantly easier to use.

However, recently things have gotten easier. The dev for i2p has included an easy windows installer for i2p that should make this much better for most users. So some development has happened there.

Also just recently qBittorrent included support for I2P in their latest release. Before that, only 2 Bittorrent clients existed. Now we're up to 3.

The biggest advantage is that you don't need to spend money anymore for a VPN. Or any money for a seedbox either if you have a home computer you can just leave up to seed for you.

The biggest disadvantage is the hard setup and (so far) lack of torrent availability. More stuff is getting added all the time but we need more scene groups adding their releases to i2p (cross-seeding).