Being nice means keeping things comfortable now. Being kind means telling someone the truth they need, which is frequently uncomfortable and always cheaper than the alternative.
Nice postpones. It says the work is fine when it is not, lets a performance problem run for a year, and tells someone what they want to hear because the alternative is an unpleasant ten minutes.
Kind is direct and early. It gives someone the information while there is still time to act on it, and it treats them as an adult capable of hearing it.
The nice manager avoids one awkward conversation and produces a much worse one later — a dismissal that surprises the person, a client who leaves without warning, a team who all knew and said nothing. The kindness is in the timing.
The test: are you protecting them, or are you protecting yourself from the discomfort of saying it?