Speaking generally: People need to remember that there are two or more sides to every story. It's impossible to evaluate any of this accurately in the context of an internet thread, and since it's a no-win situation for the person or organization being accused, there's no way for the full picture to emerge.
Speaking specifically: Peter handled my personal case with consummate professionalism and effectiveness, and I've heard many YC startups speak highly of what he's been able to do for them. YC has a long track record of helping non-US startups extremely effectively with their immigration challenges, and Peter has been a big part of that for something like 6 or 7 years. Given that track record, your story sounds particularly one-sided to me, although certainly no one bats 100 and I'm sorry that you had a negative experience.
I find it hard to believe that he would "give advice not really inline with the law"—that's a charge of professional misconduct and would be a bombshell if true. Internet comments, on the other hand, are cheap and make it easy to gunsling. Readers love drama and everyone loves an underdog, which is a vulnerability routinely exploited by disgruntled commenters. It's not easy for the other side to defend themselves (let alone answer name-calling like "snake-oil salesman"); and it's basically impossible for anybody else to tell what actually happened. You describe your immigration situation as "stellar"; whether an objective observer would agree with that is something the rest of us can only be agnostic about.