Providing negative feedback can be difficult especially where conflict, disagreement, personality clashes and negative feelings abound. For most people, it is always much harder to give negative feedback than it is to give positive responses.
For any communication to be productive and proactive all parties involved have to be on the same page. People can sometimes forget the purpose or motive behind communication and concentrate on their own feelings and agendas. No-one wins an argument ... and cause and effect dictates that when the motive is less than honorable there will almost certainly be a price to pay. You may also like to consider one of the NLP Communication Pre-suppositions that states that The quality of your communication IS the response that you get…
There is always a way to encourage a positive response from anyone even if their final decision is not to support you. No-one ever wins an argument or a heated debate and there is never really a good reason for them anyway.
If you are required to provide less than positive feedback to anyone try to remember to keep things in perspective and in context. Keep to the main points and avoid straying into areas that have nothing to do with the situation at hand.
Rare though it may be, you will find that some people, for reasons of their own, simply refuse to understand or participate pro-actively. When you have done all that you can and you have provided them with practical and reasonable options and solutions, accept that this is who they are and you are unlikely to change them.