Mastering Engineers TECHNICALLY don't do "the mix", they have to deal with exactly what they are sent...
as opposed to those of us who do both...so don't confuse the two. Also, certain things "happen" during the
Mastering phase that you have two choices...go back and fix it in the mix, or fix it in the Mastering right now,
which, being usually time constrained (time is money) it's better to do it NOW if you can and move forward.
(I'm speaking from a business perspective, of course)