What dark patterns have you observed that I should keep an eye out for?

Making it seem like you can use a premium feature, only to present you with a "You need to upgrade" view after a few steps.

Eg for Keypass and authenticator.

I'm also not sure what utility the premium features are.

There's the encrypted files, but they don't live in a vault. It seems that most obvious use case (being that you only get 1G) is to attach photos to IDs. But the implementation is silly. It's encrypted on their cloud where you download a copy and it then lives unencrypted on your device.

It seems silly that this is the implementation considering your passwords live in a local vault where you don't need a network connection.

Idk, I do want to support them but it does concern me when developers do not think about details, especially when it comes to security. The little things matter a lot.

When you enabled the browser plugin, it would completely cover the input box, preventing you from using basic browser functionality meant to prevent you from using an alternative while it's enabled.