I was reading about a famous Rav who darshened re: Avot 1:11 «Hachamim Hizaharu b'divreichem» ...
He was concerned that Mussar to a large audience can be a win-lose situation. EG some talmiddim or listeners will need Hizzuk, while others will need castigation etc.
By speaking Mussar to a wide audience, one is bound to help some and to hurt others. And some of those hurt feelings may lead to yei'ush Chas v'Shalom!
The eitzah this Rav gave was to speak of "Y'shuat Yisrael" and "dibburim ham'shivim et hannefesh"
Since I'm not clear about what he meant, I myself may be in jeopardy of transgressing this myself! It seems he meant that a positive and loving message always makes sense barabbim. It's win-win But a negative tone is a "medicine" best administered in private, when justified, lest it lead sensitive listeners to take an improper message.