After 20 years of this I still get hired on a regular basis to either mix FOH or monitors, because decades of doing both, as well as truck mixes, post mixes, and studio work, I rarely lose my sh*t at people while getting it done. I've had plenty of people come tell me my mix is ideal, and plenty of folks say "more guitar, move kick, more trombone" to whom IO say "thanks" and ignore unless they were introduced to me during soundcheck as "the person who has final say".
Most of the time, I have either worked with the artist before, or I'm there because I'm "the guy" so I am left alone and Mr. "More Trombone" is a f*kkin' trombone player.
My job is to make it sound like the record... and I can do that in spades. When it's monitors, it's make it sound like whatever they want, 110%, no "no can do", all "yep, how's that?".
Just agreed to the the monitor engineer at a festival with acts who don't travel with their own people, should be fun. I'm used to doing FOH for most festivals acts. I guess I'll set up 16 wedge mixes and 8 stereo IEM mixes on a dual-input (two idental splitters into one head) 56-channel festival plot.