In another life I was a stage lighting tech, and we did plenty of outdoor gigs in various weather conditions. It all comes down to the experience of the crew, if I alone had to run those shows there would have been similar results to Niles' Hyde park gig, but fortunately the powers that be had the foresight to never leave me in charge! There was always an experienced guy running the show who had been through it all before. Every now and then you would get a complete left field short term brown trousers moment, but to have amps overheat, or to leave vox mics off a freakin strand for the main show is just downright amateur.
With regards to the quality of the mix, it's worth Incorming the general public that the vast majority of big acts will have their own front of house engineer (the guy who mixes the PA) and even from those guys I have heard some awful mixes. I saw Ben Harper once and didn't last the whole show!
I just feel bad for the artist when it all goes down like this, it must be aweful!