password managers generate a unique password for each website. this means if one of your passwords leaks it won't compromise any of your other website logins.
nostr-login is a regression: if you leak...
Its a bit better than using a single password, since that password gets stored on many server side databases with varying security. At least with your nsec, it never gets sent over the internet to a server. It stays on your computer. Still a bad idea to use it to log into everything though.