The first important thing you should be careful about youtube videos is what you dowload from there. Some of the video are copywrited and you may have some legal problems when you download them.
Second thing you should consider is what software you use to convert the video into mp3. There are a lot of free software that do the job (a few are really good), but for some of them you have to pay. If the second case the problems will appear when you use cracks or key generator for the licence code you may have problems. The immediate problems are the viruses and malwares that usually come along the key generators (the executables are often infected with them), the long time problems are the legal ones. The software developers know about the methods to avoid paying for licence and they are creating new methods to catch you in the act – one way or another they’ll get you.