As I see it, the main benefit of deterministic password generation is the convenience of not needing a password database. Indeed, if the scheme is simple and/or portable enough (e.g., PBKDF2) you can implement it from scratch in a minute or two, depending on what software you have handy. The convenience breaks down as you need to maintain additional state: password rotation, site-specific password rules, etc.

Forgiva is based on the premise that a password generation scheme is more secure than a password database. I'm unconvinced in general; from what I see in the FAQ and the Ruby code on Github, even less so for this particular implementation. Spamming the input with an array of whatever OpenSSL algorithms Ruby happens to make available, rather than using a memory hard KDF like scrypt, is a bad smell.