Well, fair questions. So as mentioned in the footer. The service is powered by scrt.link, which encrypts the url on the client. That means br3f never knows what link you intend to redirect to. Read more on https://scrt.link/security.
Agree on the email part. Obviously you don't want sensitive information inside emails. But with br3f you have additional security, since a link can only ever be accessed once.
Why would I trust "br3f" with "sensitive URL"? Why would it be accessible by anyone or from anywhere to begin with? And most importantly, why would I put it in email which is literally the least secure form of communication?