I think that's backwards -- if it's a very accurate simulation of water, it will pass the U test, but you can add hacks to make a simple, inaccurate system pass it, too. See, for example, Dwarf Fortress's pressure system: http://dwarffortresswiki.org/index.php/DF2012:Flow#Fluids_un....

I agree, the U test is very hard. I spent many days trying to make a working 2D approximation with cellular automata, but I couldn't find any way to make the pressure propagation fast enough. The water would eventually level out on both sides of the U, but it took a very long time to do it.

What happens instead in the simulations that fail?

usually the water fills up the bottom part of the U until it reaches the width of the pipe, the only one side fills up (the supply side). So if I fill up the left side of the U pipe, the water forms an L shape. On some simulations, the other side rises, but doesn't really even out.

they seesaw for far longer than is reasonable.