That somewhat implies there are a lot of places that don’t accept Monzo as is. Now, aside from a few random incidents (can happen with any card, to get a cashier on a power trip who doesn’t understand they must accept all valid cards and randomly picks some they don’t like - it’s rare but it happens), a few merchants forcing 3-D Secure to succeed and maybe the random other technical issue caused by various little things, is there really an epidemic of Monzo card acceptance problems?
My experience is that it’s well-accepted. Not as good, perhaps, as most UK-issued Mastercards. But better than American Express (and I mean at places that theoretically take Amex - they have massive compatibility issues. For example Wilko, Hotel Chocolat and Superdrug don’t accept Amex contactless - a list that used to be much longer). And better than online PIN (with signature in 2nd place) or signature preferring (mostly foreign) Mastercards.