My recent experience is only with AA AAdvantage. Three biggest problems:

1. Getting consistent information from phone agents about program rules as they apply to your situation of the moment. Call 5 times and sometimes get 5 different
answers. Your only dependable recourse is to keep searching until you find something in writing. Flyertalk has been more helpful in finding out what the rules are than the telephone agents are.

2. Using upgrade certificates (called VIPOW's). Even though they are confirmable, they seldom will confirm when you make your reservations (even 6 months ahead) and make you wait until they're absolutely, positively sure they can't sell the seats - even if the seating chart the day before the flight shows the cabin is basically empty. Then they usually give it to you 5 minutes before takeoff.

3. Gradual inflation of mileage requirements for awards.

Otherwise, the program has been good and given us some great opportunities.