I would much prefer an option in the app to “Generate online card number and CVV for [X] usages/up to [X] amount” - so you can give each merchant a unique card number and CVV and limit how much it can be used. I know about a decade ago, some (particularly US based) banks were trailing (see https://www.bankofamerica.com/privacy/accounts-cards/shopsafe.go and and appears to be the same idea as https://getfinal.com/ ). Then any unauthorised usage is very very easy to track down and it is a lot more secure.
The disadvantages are that Monzo would need a lot of “spare numbering capacity” and if a merchant asks you to confirm the last 4 digits or the CVV number on future purchases (such as the National Lottery does online), you’ll have to have a way of finding/looking up which card combination was used.