Monzo/Mondo had issues on their own, and in fact they have very frequent issues with their payment processors.
The current account (now the only account as the prepaid beta programme has ended) runs on their internally developed payment processor, and has so far had minimal issues.
More information here: https://monzo.com/blog/2018/04/04/ending-prepaid/
I know someone who interviewed at Monzo and said the interview process and technical test (write a web crawler in Go) was generally a pretty good process. However they "noped!" out of there when the interviewer said that they don't do unit/automated testing, and it's the developer's responsibility to make sure their code works.
This was a while ago, so hopefully things have changed or hopefully the interviewer was not telling the truth (perhaps looking for a reaction?). However if it is the case then they're about as "modern" a bank as all the rest of these legacy banks, or at least will be in a few years time when their technical debt starts to catch up with them.
As far as I know Monzo is using microservices (a bank is probably big enough for that to be the right choice) to try and limit the damage of anything insane like this. The way they handled Prepaid to Current Accounts was absolutely seamless https://monzo.com/blog/2018/04/05/how-monzo-to-monzo-payment...