Saying the mx980 are bad for vocals is like saying the sony XB series is bad for bass heads.
It is a massive contradiction and makes no sense at all. The XB series has powerful clear and airy bass, just like the MX980 has incredibly clear and wide sound that projects vocals unlike any other earbud ive ever used in a non-inner ear style.
The mx980 were designed specifically for great vocals and a deep bass experience. They are not meant to be a musical set. They are of a higher quality and very neutral sounding, providing for a rich, wide vocal experience. Intended, at least i think, for acoustic guitar, light classical and live jazz vocals due to its staging shape, which is a rectangle that is not so tall but very wide.
so, when someone says they sound awful for that, I really dont know what else to say except you have a defect.