Which? ranks Monzo last for app security

Password reuse across multiple accounts, is I believe what is meant here. If every financial service requires multiple passwords, a user is more likely to create one set of passwords and use it across all their services, rather than create a separate password for each one.

This means that a service that initially may look more secure on the face of it leads to a situation being created where if one account is compromised, then all accounts can be compromised.