[septimus111]

"Directness" is an excuse that I sometimes hear to justify aggressive or rude behaviour, but it's possible to be direct in a polite manner. "This new plan is stupid" is rude, whereas "I'm frustrated about this new plan, it won't work because X" is assertive without being rude.

[brailsafe]

I disagree that your alternative is assertive, and that the former is inherently rude. It's only rude depending on each person's social communication expectations. Otherwise, it's an assertive statement, and like any other, it could seem rude. I find it rude when people are passive aggressive, but they might feel like it's polite.

[bartread]

"The new plan is stupid" is rude because it's not actionable. It doesn't say what specifically is wrong, why it's wrong, or how it could be better. The older I get the less tolerant I am of people simply venting or breathing out generalised criticism that doesn't help us navigate to a better place.

I expect people to bring criticism, but I expect them to do so constructively if they want to be listened to. "This plan is unworkable because X but if we did Y instead that would lead to Z which would work much better," is actually helpful. "This plan is stupid," isn't.