I don't have a problem with fab rest it freely rinses out when I clean upholstery, I can see through the hydro filter as it runs clear. Although different fab rest and ultra pak are from the same family of detergent based co solvents so if you have problems with one you are sure to have problems with the other.
Most problems are usually from the technician not the product, if the cleaning company have under cut your price then it goes to show that they have to cut back on other things. If there is a residue problem it doesn't necassarily mean that it was the prespray as it could be the rinse detergent or if they have used a post deodoriser.
Shaun