Uneven ceiling, best way to fix it?
In many cases a ceiing in an older home if replaced needs to have an attic strongback installed to occomdate the new ceiling.  As a home shifts with settlement of ground and foundation the current strongbacks are compromised.  So when installing ned to old, the old ceiling joists need to be supported.  When the old drywall was removed, the old strongback releases pressure and caused the joists to shift.  So work must be done to the structure of the attic.  It is obvious that the strongbacks and the joists are drooping. Is it the drywall guys fault?  Yes and no.  He is not a structure contractor.  It is the General Contractors fault.  And if the home owner was the GC...it is the inexperience of the homeowner.  When you hire a finisher to repair problems, you have bad results.  In a court situation if you attempt to litigate, the finisher will win the suit as he is there to install the drywall to the existing structure only...He does not have the skills for anything else.