But = 'That’s great, now this is what I think'
Discredits everything said before it and gives the user the chance to railroad them with their own ideas. Always replace with the word 'and' however uncomfortable that may seem.
e.g. "You have clearly put a lot of work into this project and I am very grateful but we have decided to drop it for another initiative".
... he was saying 'you have put a lot of work into this' - which on the face of it sounds like setting a good leadership example. Then the word 'but' arrives, essentially allowing the speaker to wipe away everything that they have said before and steamrolling it with their own approach, ...