<<< There have been a number of high-profile cases in which
contract fund-raisers have swallowed up in fees and lavish expenses
almost all of the money raised, though I do not think there have been any
allagations against museums on this point.>>>

I don't understand.  If the fundraiser was working on a percentage how
could he have used up all the money raised?  I can see this happening if
he charged a flat fee, if he charged $5,000 in fees and the drive only
raised $5,000.   Actually I don't see why a percentage is all that bad.
If the goal is $100,000, and the fundraiser charges 10% or $10,000, with a
cap of $10,000 maximum fee to be paid, then if the fundraiser is not
performing his job correctly he would not recieve full payment.

Charging a flat fee gives the organization little means to entice the
fundraiser to reach the goal.

Where is the hidden pitfalls with percentages that I am not seeing?

Rick