Ok so there are 4 main variables that you have in any live sound situation (my opinion of course)
1. the room (not applicable if the show is outdoors, but wind can be a factor at outdoor shows depending on your distance from the PA)
2. the equipment
3. the band
4. the skill of the engineer
If any of these one things is lacking, it could destroy the mix. I mixed a metal show in an old church a couple of months ago, and just imagine trying to mix a modern metal song that is coated in 80s style cavernous, natural reverberation. It didn't help the fact that the bands wanted their guitars blasting at 110dB from the backline and even louder from the monitor wedges. The bands that had the quietest monitor mixes had the best FOH mixes; some rooms don't have this issue but you sometimes it is necessary to adjust to where you are playing (ie side washing guitars in smaller venues).
Guitar volume is especially problematic at smaller venues and can easily ruin a mix, because often times these venues don't have enough PA and the guitars don't even end up in the mains because you have to use every ounce of that power to get the vocals above the guitars. Some guitar tones just won't cut; no amount of EQ can bring in mids that aren't there.
If you're in the crowd, a lot of the time mix issues can be solved by moving throughout the venue until you find the "sweet spot." It'll probably be near the FOH desk, but not always. If you think the subs are too loud, you can always move back, because we have to mix for the people at the back of the crowd too, and that sub has to carry a reasonable amount of distance. If you can't hear the vocals and are near the front of the stage, move back.
Most of the tour guys I work with tend to be pretty good; I'd say about 70 percent good to 30 percent bad. Anyways, enough rambling, but everyone makes it seem like it's always the engineer's fault, but the engineer can't upgrade the equipment the day of the show, change the room to have ideal acoustics, or teach guitar players to have good tone
However, there are some dudes that just plain don't give a shit that make everyone else look bad, but I think you have that with any profession, and that's why those guys work at shit bars without adequate equipment.