I'd say rap sucks. You can't even understand half of what the criminal rapists are saying.
As a death metal fan, let me respond by saying that understanding the lyrics is not always that important.
Then why even have lyrics
1. While not everyone may understand the lyrics from hearing them, some may.
2. Anyone is free to read the lyrics, after which it usually will be much easier to understand what is being said.
3. Vocals can be used as an instrument. Even without understanding the words, the vocals can add to the song.
4. Sometimes one might understand only parts of the lyrics, such as a chorus, and that can be enough.