Being fake nice is just being rude on a delay.

Pleasantness laid over something else does not remove the something else, it only postpones it. The other person usually finds out what you actually thought later, from somebody else, in a rougher form than you would have given it. The delay adds the discovery that you were managing them. Honest discomfort now is much smaller than that.

Choose the kind true version over the pleasant false one whenever something difficult needs saying. Keep it short and keep it private. People forgive bluntness far more easily than they forgive being handled.