You are responsible for informing me of any irreconcilable problems within your group. I will try to help you resolve them. Do NOT submit any written document to your client until I have read and approved it. Use figures and graphs to make your point. Don't waste anyone's time.
Short reports are okay.
Get right to the point and stick to it. Talk about the client's problems, not yours. Defer all data analysis, details of models, etc. to an appendix. This is not reassuring to a client, who assumes you are doing your job professionally and will only become alarmed at claims that are uncalled for.