In my opinion, there is a huge distance between politely pointing out the faults and problems with something and sugar-coating.
And maybe this approaches the crux of the problem. It seems that there is a false dichotomy at work here.
Being polite (or, indeed, just not being verbally abusive) takes less time and energy, and is overwhelmingly more effective, than being overly abrasive.
I don't believe 'Sugar-coating' has anything to do with being polite and accurate. 'Sugar-coating' is all about being less accurate and on point.