Chilli - pre-covid we moved across to IEMs and with personal mixers, had the best mixes ever - BUT - at festivals it went horribly wrong. Festival sound guys with few exceptions are old rock and rollers, or young people first outing in the industry and their monitor mixes with wedges are at best average. I realise there will be some excellent guys out there, but we just met few of them. With wedges if your mix is totally screwed you can usually hear enough to play - a struggle but you get through. The commonest thing is they are so far away, they cannot tell who you are or even what role in the band you have, so can I have a bit more keys in the mix please probably gets the keys players mix changed not my bass! With IEMs it's intolerable. With them in you really need a balanced mix and a quick line check is NOT going to work. What we did (for 5 or 6 shows before Covid was buy an X32 rack and stick it in our usual stage rack. We rigged a festival system. We could pre-rig our 4 vocal mic cables with a Y splitter and drum overhead plus bass, guitar and keys feed - just 8 mics/DI and we and our sound guy we always take with us could unplug their 4 vocal mics, add our splitters and replug, plus the DIs, and one boom stand we put over the kit. This gives us all our vital feeds and the stage or FOH festival people get their pre-labelled sources. My bass amp and the guitar amp have DI out, and the keyboard just needs a DI box. It works really well. It means we're totally independent - our personal mixers are network cables so we can get rigged quickly, ready to play because we don't need to do our mixes.
I find the IEM mix in just one ear very difficult to play with. Festival stages are only good with IEMS if you have proper soundcheck time and our band have four part harmonies in every song - and in my case, if I lose the keyboard channel, I have no idea if I'm in tune. At one festival the stage level was criminal and I played one entire song in Eb, for getting we'd decided to play it in E for a change. I did the whole song wrong - but never heard a thing as I'd pulled my ears out because the mix was so awful. I play my bass by remote control, so could still play the right (but wrong) notes without hearing them. I only knew when the sound man told afterwards. I've even played a gig with the ears in and the pack off because it was so loud.