they do work as intended. the main benefit is that your engine won't lean out during the flight, where you set it at a full tank is where it will stay until it runs out of fuel. it works because when the tank is full, the pressure inside will be less then what you would have otherwise, the muffler pressure is having to fight the column of fuel to push air into the tank. (imagine blowing bubbles with a straw deep in a cup vs at the top) when the tank is nearly empty the weight of the column of fuel is much less, so more pressure can build up, compensating for the lower fuel level and keeping the fuel pressure roughly equal.

just test ran my shrike with a irvine .53 and a uniflow setup. one interesting thing is that after running at full throttle for a little bit, you will get a little fuel going back into the pressure line when you suddenly chop the throttle. at that point the tank has excess pressure and it has to escape somewhere. (probably just going to add more pressure line in my situation so I'm not dumping fuel out the exhaust)