From the Apple App Store regarding Safari settings:

This extension can read and alter webpages you visit and see your browsing history on all websites.

This includes sensitive information from webpages, including passwords, phone numbers, and credit cards.

The whole idea sounds great, even reasonably priced but... having to trust another company on all that seems too risky. They should offer a standalone browser instead of a Safari extension, so that the user can easily skip it when dealing with sensitive data.