the pressure change alone can have an impact on things like oxygen levels (they're lowered), can cause the gases in your body to expand (e.g. bloated stomach, gases in the bloodstream expanding etc), causes dehydration, affects the eardrums, increased risk of nausea/motion sickness...

on a minor level, there's an increased radiation level at higher altitudes, and a minor risk of something like deep vein thrombosis although even a tiny swelling in the legs/ankles could potentially cause a more significant injury to the player.

hell, if i were only going to go to a gig in london i'd personally prefer to travel up the night before and have the day there before the gig to relax.