So, myself & my wife have a Monzo account each, and now a fab joint account.
Today we went on holiday flying EasyJet from Luton to Dalaman, Turkey. Budgeting all done, Monzo was our card of choice. Not on EasyJet. We came to pay, the cabin manager declined to accept our Monzo card… citing an internal memo ‘Not to accept those bright coloured ‘mombo’ cards (made me laugh)’ asking why he said they don’t take pre paid cards, of course I said it was a full bank account, but he was having non of it … hang up from the past maybe, but an issue non the less.
Needless to say we had other means to pay, but if we didn’t… that would have been a dry flight 
I’ve read historical forum posts where this was an issue, but maybe Monzo need to reach out to EasyJet if that’s not been done already