I think anger is ok and justified at times, and not always just a consequence of being in the wrong state of mind, etc. I think we're more and more moving to a mindset where anger is just not really ever acceptable, either to feel or to display. But, that's all a bit heavy for what this is trying to accomplish, which is certainly an understandable goal.
Anger is definitely not okay. Ideally, you say - 'This project isn't working because you failed to gather requirements from the right users." or "You spent three months choosing a vendor without proper due diligence who doesn't have a working client library, documentation, or a functioning test endpoint". However the proper way to express anger in our enlightened business environment is passively and non specifically…
Both of those statements are completely reasonable. Don't conflate directness with anger, berating, personal attacks, etc. It's entirely possible to be direct without attacking people.