Assuming your master key/password is strong this system is still secure if one the sites you use leak the generated password. However, if you somehow leak your

Assuming your master key/password is strong this system is still secure if one the sites you use leak the generated password. However, if you somehow leak your master key then all your accounts are exposed. It's weakness mainly is if the machine you use is compromised so are your passwords. However, the same applies to a password manager. Unless you use an offline system to generate these sub-keys.