You never really know until you walk a mile in someone's moccasins what they're really dealing with....
It is always easier to know what would improve the mix when you're not the one mixing too. I tend to jump to thoughts myself about what the engineer could be doing better or what seems wrong at events I'm not working (like we probably all do). But in reality I don't know what they've already spent hours trying to conquer to get to the point they're at with the mix or the rig as a whole.
This is so true and I've been guilty of it myself. The light bulb went on for me after a church weekend I mixed a while back where I spent about an hour during practice/sound check trying to get the toms to sound right. I tried changing tom mics, changing positions, all sorts of gating and EQ, but nothing worked. Because of the way that drummer played, there was just too much cymbal bleed in the mics and pushing the tom mics louder just turned the whole kit into a muddy mess. I ended up going with 90% overheads (for the toms) and 10% tom mics to keep some coherence in the overall kit. Never could get the beefy tom sound I wanted.
The services went fine that weekend and I didn't think anyone else would notice (or care) until one guy came up after one of the services and introduced himself as a sound guy from another church and asked why he couldn't hear the toms very well. I explained the issues and he seemed understanding, but it really opened my eyes to when I go places and things don't sound "right". I tend to be a lot less concerned with less than perfect sound from other engineers now because of that reason.