All the so called 'neo banks' are still in their infancy which is good reason to avoid them at all costs. I'm waiting until these startups mature (and they are startups).
In the UK, anyone with the banking license Monzo has is protected by the insurance mandate which was put in place after the collapse of Northern Rock. The service from Monzo is substantially better than any other British bank, so it's a risk I'm willing to take.