If you want to get into the reeds of it, I think "assertive" and "strong" are too positively loaded (which I guess is why you didn't use them). However I would say the same applies to robust, coupled with the fact that "robust" on its own is not really applicable to the kind of communication you're referring to. The positive undertones are probably also why you saw a bunch of unnecessary knee jerk virtue signaling in response.

I would personally have stuck with just "direct" (which is neutral), and perhaps added "blunt" (which is neutral-negative).