Here are some remarks from Better-Bottle's website with regard to washing them, and how brushes should never be used.
Hot water and the right detergents do an excellent job without scrubbing. Never use stiff brushes, abrasive scouring pads, or cleansers.
Detergents recommended for winemaking and beer brewing do not damage Better-Bottle carboys and related components when used according to the manufacturer's instructions (See Detergents below). It is rarely necessary to remove a Racking Outlet from a carboy in order to clean it. Patiently soaking a carboy in a solution of low-suds detergent and hot water is an easy and effective way to loosen stubborn contamination, which can then be rinsed out. For faster results, put a few cups hot wash solution into a carboy, add a face cloth, and vigorously slosh the solution and cloth. Better-Bottle carboys are light, so sloshing the carboy in your hands will not be difficult; however, rocking the carboy back and forth on its side or rocking it on a tennis ball placed under its punt requires even less effort. At each stage of washing a carboy fitted with a Carboy Adapter and High-Flow valve, the valve should be opened to allow some of the solution in the carboy to run into and though the valve.
Better-Bottle carboys are more easily washed and sanitized than glass carboys, because Better-Bottle carboys are so light weight and the surface of the special PET from which they are made is non-porous and hydrophobic. Hot water, a good bottle cleaning detergent, and some soaking will separate solids and films from the walls of Better-Bottle carboys without the need for brushing. Better-Bottle is not aware of any home winemaking and beer brewing detergents or sanitizers that damage Better-Bottle carboys and fittings, when used according to their manufacturer's instructions.
Stiff brushes, abrasive scouring pads, and cleansers should never be required, or used.