Well, I have used other similar websites and apps and I have experienced the same things on all of them.
People to whom you send a message and do not answer.
People that answer just one or two messages and then disappear.
People with whom you were having a good conversation and, all of a sudden, they stop messaging for no reason.
Among other things. It not only happens on Interpals, this happens everywhere else. I guess that knowing this beforehand will save you a lot of trouble. Still, I was able to make some friends like that. It may be luck, but it is possible. You may also not make any friends, but at least this site allows you to have some good conversations with people, even if you don't continue talking with them in the long run. Just like in real life, you don't continue talking with many people even if you considered them friends.
Overall, I think the site is good if you know how to use it, despite its flaws.