I don't understand the reason for telling clients there are difficult. I have some very difficult students and I'd tell them that we need to work on XYZ and set up XYZ goals and get better at XYZ but I'd never tell them " you are difficult". It's rude

As about concept of being difficult as clients. I do not understand that but I understand concept of difficult people.

Some people are just difficult no matter in what context: confrontational or argumentative or always negative. Some are just jerks and some are this way due to mi or special needs etc it's ok to address their behavior and help them to get better, but simply telling them that they are difficult people is pointless and unkind.