The answer is yes, and it depends.
If you’re a beginner and you want to learn crazy advanced submissions, then it’s a bad idea. But youtube instructionals can be really helpful as long as you don’t try to learn too many new techniques at once, or techniques way above your level.
It’s not nearly as helpful as a good instructor who can teach you the technique and then watch you do it and help you do it correctly.
One of the best bjj youtube instructionals is Kurt Osiander. He is very entertaining and the techniques he does are fundamental, high percentage techniques. If you are offended by cursing or have little ones around, don’t watch Kurt Osiander!