He quotes from their site where it mentions that their goal is to "continually evolve by attracting a community [...] to provide additional functionality to the already existing rich feature set."

Of the five raeasons he gives, here are a few:

• difficult to integrate into an existing design
• admin navigation issues
• security

While some of the issues her points out here are a matter of personal preference, the "security" aspect caught my eye. These days, especially with all of the talk about security in the PHP community, that should be one of the first things groups look at in their software. From his comments, OSCommerce doesn't make it very easy to implement a patch either.