Shader compilers are pretty bad an result depends on browser and driver. There is plenty of platform differences when accessing uninitialized memory, using more complicated constructs like structures, performing math operation with undefined result, incorrectly defining if function argument is in,out or inout.
The shaders on https://shaderfrog.com/app all work fine for me and those looked pretty complex. Maybe they're not using the same type of "shader compilers"? Or perhaps making a CPU emulator with shader compilers is just more complicated...