I've flown and maintained large radial engine powered aircraft that utilized avgas, drawn from both integral fuel tanks, and bladder tanks. The same concerns exist there, and the same characterisitics are present. It becomes a matter of pilot awareness, rather than a design flaw of the airplane. A poor carpenter blames his tools. The rest learn to use them properly.
Many people fire up their airplane and taxi to the pumps, before sumping the system.
Having done this, they have already drawn moisture into the fuel feed lines, and cannot further sump it. Any sumping following that engine start is going to miss most of the moisture. It's far better to tow or pull the airplane to the pumps, if the pilot was neglectfull not to fill the tanks before putting the airplane away. Sump before the two, sump after. See if more moisture shows up.