I've fronted a few bands in my time.
The odd thing is I find it very easy when I'm playing guitar, but quite difficult when playing bass.
I don't know why - I'm a far better bassist than guitarist but there is something rhythmically that seems to have vocal and guitar fit together easier. For me anyway.
The complexity or speed of the music doesn't seem to matter either. I struggle with bass and vocal on "Sittin' on the dock of the bay" but I can nail anything Metallica related on guitar and vocal.
I hate my brain sometimes!