Dr. Julia Garner in a conversation with Jim O'Schaugnessy on the Infinite Loops podcast on the critical distinction between being nice and being genuinely kind:

There's a big difference between being nice and good, being nice and kind. The differentiation is that nice is about pleasing other people and about other people's response. Nice people are often not genuine because they will do things to make the other person feel better that isn't authentic to them...

Good, kind people hold lines and have hard conversations. Nice people don’t have hard conversations because it's not 'nice' to do that—it makes people feel uncomfortable and put out, and you have to sit with that.

Love this view of the distinction

Being "nice" often masquerades as virtue, but at its core, it is conflict avoidance and image management, prioritizing short-term comfort over long-term truth.

Kindness requires courage. It means caring enough about an outcome or a person to endure the immediate discomfort of setting boundaries, delivering hard feedback, and holding the line.