I'd say, putting aside quantifying what instrument is more important, that vocals are perhaps the most difficult to discern while recording as far as monitoring goes. If all else is equal, the monitoring of every instrument while performing and recording is a more homogenous operation. A vocalist has more to discern because his or her execution and precision depends on what they hear and that's all internal; in their head. For me, playing guitar or striking percussion or whatever is external and I think easier to execute without hesitation. When singing, especially metal type of vocals, you have to be loud and exert power while perceiving this inside your head you've got your cue mix or reference cranked as well. The louder you are, the louder the cue mix has to be. It is somewhat more of a frustrating and potentially stifling process as opposed to, as mentioned already, the more natural setting of singing out loud without something on your head, muffling your hearing or "being in your own head" while trying to be precise and inspired.