The thing with mixing guitars is that the frequencies they do need - for fullness, weight, or presence -  are some of the same midrange frequencies that are most crucial for vocals. Some guitar parts inherently sound good a little thinner - think funky rhythm guitar playing, for instance - so this is less of an issue. But a lot of guitar parts, especially distorted rhythm guitar tracks, need exactly the same frequencies where we find the fundamental notes of vocal parts and crucial early harmonics. This can create problems. Maybe you’ve encountered them.