Having actually done live sound for large venues I can tell you that at best it is extremely difficult, trying to work with the acoustics of the venue, the artists (onstage monitors are a completely different mix with different objectives from the house mix), the road managers who love to backseat drive the mix, often with little or no regard for overall sound quality. Add to that most sound guys have little or no formal training in sound reinforcement, architectural acoustics, or even basic listening. There are notable exceptions, many already mentioned, the last gig I helped produce was in the chapel at Bastyr University outside Seattle. We hired Morgan Sound for the job, and when they learned the venue, they assigned one of their 'A' crews. Bastyr Chapel is long, narrow, tall and very reverberant. It required 4 speaker sets, the second set delayed about 60 msec to account for the distance from the primaries. They pulse tested the rig to set the delay, and only eq'd using the singer's voices to get a natural sound. The result was exceptional. These guys were pros. More recently I heard the Mavericks, and their opening act sound was hideous, both muddy and harsh, basically a mess. The mixer clearly had no idea what they were doing. The Mavericks were a revelation, same gear, same venue, different hands on the controls. controls. They sounded great, and the volume was appropriate.
Thevbest recorded example of how tough live sound can be is the the track "Stay" from Jackson Brown's " Running on Empty". At one point you can clearly hear the system being pushed hard by the performance and the vocals start into feedback, only to be caught just in time by the monitor mixer and pulled back, into control. Believe me, because the show was being recorded, the mixer saved himself an uncomfortable post-show moment, and maybe even got a thanks for that. The result on the record is the the band, the vocals, and the system all giving everything they have for the music, and it's there for all to hear. Some days it just all comes together like that.