I've had some issues with it but I have used it (testing to onboard new people mostly)
On the nsec.app page make sure notifications are enabled.
The actual log in is a bit different depending on the app you're logging into. You can try putting the nsec address for the login, go back to the nsec app and there *should* be a notification on that page to enable that app.
I've had more luck with apps that support the bunker urls (nsec.app -> connect app -> copy the connection string). Some examples of apps that have worked for me are: https://coracle.social/ and https://nostrudel.ninja/