YouTube beats are a big source for instrumentals for a lot of upcoming rap artists these days. They are cheap (sometimes free) and easily accessible. Chances are if you are reading this you may have used them. However, they sometimes come with a cost. Pre-mixed instrumentals found on YouTube tend to tie the recording engineers hands when coming to the final mix. These beats are almost always mixed down by the producer of the beat, and very rarely are they trained at mixing. Even if they are, their job is to give you a mix that sells you the beat, not one that is necessarily best for blending vocals into. Quite often beat producers mix with loud instruments dominating the verse leaving no sonic room for a vocal, and in the end over compress the beat so that it sounds good to the artists browsing through beats online, but making it difficult for the recording engineer to mix and master properly once the song is done.
Buy your beats from someone who can also provide you the multi-track stems. This means that instead of a single Wav or MP3 file you will receive a bundle of files resulting in a single file for each instrument. While this is probably going to cost more to purchase, your end result will almost always be better. It’s going to give the engineer the ability to mix and master the song properly, and not be forced to deal with what has already been done.