Heli....there's some folks that post here that can far better answer your questions as they were involved in the Chinook operations and some in the Bristow decision process that led to the Puma/Tiger/Super Puma decision.
Costs, lack of uses in other kinds of tasking, vibration levels, are among some of the complaints but one must also remember the marketing campaign (or anti-Chinook campaign as some believe) that painted the Chinook in a bad light as being part of the reception the old girls got on the North Sea.
There have been some recent discussions about this in Rotorheads....cannot recall which thread.