My starting point would be that 'nice' and 'kind' are not synonymous.
Nice - generally prioritises not wanting to hurt someone's feelings, never wanting to say something 'mean', wanting too be liked, wanting to help regardless pf the impact on themselves. So someone effectively has to lie (to themselves and others) in order to achieve it or disadvantage themselves in some way (that's why it's seen as a weakness by some - a nice person will never say no).
Kindness - generally prioritises respect and honesty to both parties.
Imagine I am in a relationship (romantic, friendship, professional). I really don't want to be in that relationship anymore. I don't enjoy their company, they might be disrespectful towards me, I might just not fancy them anymore, their values don't align with mine, I don't think the service they provide is up to scratch - whatever.
If I were a 'nice' person, I would continue the relationship. I don't want to be seen as the bad guy, I don't want to want to hurt their feelings, I don't want to want to let them down.
But I grow increasingly resentful. In a romantic relationship, this might look like snide comments, no sex, irritation accompanied by guilt. No fun for either person. But I haven't said anything to them because I'm nice...
In a friendship, it might look like a slow fade - fewer phone calls, messages or catch ups. Leading too upset and confusion on the friend's part. Or ignoring my own feelings and just feeling irritated and annoyed by them. But I haven't said anything to them because I'm nice...
In a professional relationship, it might mean I know my windows aren't being cleaned but I pay them anyway, I don't like the way my hair looks but I've been going to this hairdresser for years and I feel awkward about changing. So I carry on. I have negative thoughts towards them and the professional service they provide but I carry on. I might moan about them to my friends or start a thread on MN to complain. But I haven't said anything to them because I'm nice...
However, if I were a 'kind' person, I would end the relationship. I would consider the impact of my words on the other person so I would do it respectfully and not with any intention of causing further distress. I would understand that people can feel upset or disappointed or angry without that meaning I have done something wrong.
We are both able to move on and find more satisfying relationships elsewhere.
Nice leads to people pleasing behaviours. Other people know that they are a pushover and will never say no, even if they want to. People lose respect for them because it's hard to respect someone who doesn't respect themselves. Nice becomes a weakness because it generally only goes one way.
A kind person has boundaries. They will help someone else if they have the capacity to do so but also feel comfortable saying no. Kindness goes both ways.