Hey
I guess like most instruments, to do either well is difficult. A lead vocal does tend to be under more individual scrutiny, and balancing expression with precision can be difficult although a great deal depends on the genre. Almost always a lead vocal has to have a stand-out feature that gives it a distinct and likeable character.
Singing a harmony vocal can be really difficult too. Achieving a blance in tone, and volume, precision in pitch, and synchronized timing. If anything harmony vocal tend to be warm but reasonably characterless. By that I mean that they tend to be of sympathetic tones. Where harmony vocals do display more character, it is commonly a dual vocal, more than a harmony vocal.
imho...
:
Cheers
John