I have this friend who is extremely smart, knowledgeable about a number of things. He will share his knowledge freely. He will admit when he does not know something. He welcomes and is willing to take advice. By all measures, he is a great guy and will drop everything to help you out.
The problem is that his promises to complete and deliver critical tasks are just not reliable. If asked about his previous promise, he will reply, "life happens." Unfortunately, this has cost him a number of lucrative business opportunities. Sometimes it is better to say no than to damage your reputation and becoming known for your lack of reliability.