"Mainline DHT" is the primary one. It backs magnet links and has resisted censorship for over a decade. Largest most robust group of cooperating computers there is.
You can reliably use it to store arbitrary key-value pairs up to 1kb in in size.
You can reliably use it to store arbitrary key-value pairs up to 1kb in in size.
The "break point" is when a DHT gets big enough I can't realistically MITM all the links with nodes "closer to the target" than existing ones.
This means big networks are great, small ones are cheap to just break. Its hard to skip the messy bootstrapping phase.
I'd encourage protocols to only rely on DHTs for small key-value stores if there isn't a trust mechanism in place to validate new peers.
Otherwise, all I have to do is mine for O(n^2) dht keys that cover the network. Figure out what your key mining difficulty is and you can identify what the cost would be.