I had good experience with Columbia Asia + AIA. Even though the doctor said during pre-surgery consultation that the procedure might not be covered (just try to submit for GL), it was approved and they paid the entire sum of surgery including post surgery care 60 days.
But agree it is unavoidable for private hospitals to mark up certain things when they know is under insurance. So far Columbia Asia is still reasonable based on my experience.

Agree. The doctor need to provide the accurate details when requesting GL. And if there are things that might not be covered, the doctor need to inform the patient beforehand that they might need to fork out own money if the final amount is not settled by the insurance co (even though GL is approved).
That's what my doctor did.