Getting a good FOH mix or hearing yourself on stage can sometimes be tough, no matter how good your audio system is. Some rooms are just really crazy sounding, and there's not much you can do to fix it.
This is especially true of larger venues. There are some arenas, like Madison Square Garden, that actually sound pretty good and are fun to mix in! But many arenas are really designed to sound LOUD for sporting events, and this definitely does not lend itself to live music.
Even smaller venues can sound weird. Some are boxy, some are TOO dead, and some just don't have enough of a sound system to compete with acoustic flaws. You're not always going to find ideal conditions from room to room.
But, there are definitely certain venues around the world that have great stage sound, and these tend to be the ones where performers have great shows. A good audio system will help with this for sure, but a favorable acoustic environment helps a ton.
This is especially true if you're using wedges. When using IEMs, the acoustic environment won't have as much of an effect on the mixes, since IEMs are a much more direct way of hearing the sound. But, the sound from wedges will bounce off any surface area around the stage.
The sound of a room will also change from sound check to show time, as people fill some of the empty space in the room. Usually, this is a good thing, and sometimes terrible sound checks will end up being totally fine once the room fills up.
Anyone know of a particularly difficult room to mix in or play a show in?