Standard issues with these derived password schemes:

* Are you sure your algorithm can't be reversed?

* What do you do if your normal username is taken?

* What do you do when the site's name changes?

* How do you handle forbidden and mandatory characters?

* How do you handle forced rotation?

* What about extraneous crap like security questions, phone PINs, emails, related sites, &c.?

* How do you access it on other devices?

* How can you track down old accounts to close them down?

If you go on listing the issues, you wind up writing the requirements document for a password manager.