Is power flush auto transmission over 100K miles bad? on 1999 Jeep Cherokee

My Mechanic told me that power flushing an automatic transmission with over 100K miles will cause slippage in the transmission because "the gunk is what is holding certain parts together and is you remove the gunk they will not work like it used to" Is this true??? or is it he just does not have the equipment to do a power flush and he just does not want to loose an easy $100 ??

Asked by for the 1999 Jeep Cherokee
He's probably right as most transmission mechanics say that if the trans was not serviced every 30,000 or so miles (fluid and filter) then don't power flush the system as it will knock the varnish and debris loose and could cause problems