• the back-end code being old and hard to understand
• the front-end code being overly complicated with the custom template language and the need for compilation

• in my experience it is faster than most competitors, even with big forums
• the API is somewhat old but extremely powerful

I would say that I am about 50% done with alpha quality code.

Almost there, just finished implementing user registration.

Funny, I though I had removed that call. You can replace the offending line with the following:
$data['html_body'] = $this->parser->parse('fud/post.php',$data,true);

Be aware that smileys will not render correctly (that fix_relative_urls function was there for that purpose).