I have been working a theory about waterproof boots for the last few years, but I have never heard it mentioned anywhere, so maybe I'm wrong, although I think it is sound.
For years I worked and recreated in modern style liner boots and they always seemed to fail at some point fairly early in their life. After I moved to a winter climate, I was using Keen Polar winter boots for outdoor stuff in the snow. They were brand new and every time I went out to do something in the snow, it felt like my feet were getting soaked. Sometimes my feet would be wet when I took them off, and other times my feet were dry, despite feeling wet when I was outside. I started noticing that when they were dry was on colder days. I started thinking about the dampness being all sweat and not actually leaked-in snowmelt; colder days, no sweating=dryer feet. So I started wondering why it always felt like my feet were wet. These Keens were mostly mediocre leather, but the tongue was synthetic material with a somewhat coarse texture like canvas.
Then it hit me: evaporation/melting! Ice/Snow requires energy to melt into water, and water requires energy to evaporate. Both of these processes suck energy in the form of heat from nearby sources. This is the opposite principal of why ambient temperature tends to rise when it rains; condensing water releases stored energy in the form of heat into the environment. Both of these processes occur in our refrigerators constantly: liquid is evaporated to suck heat out of the interior of the fridge, then compressed so that it condenses (releasing waste heat) and can be cycled back for evaporation.
I started wondering about that coarse synthetic tongue, which, even when freshly waxed/oiled, trapped water or ice on the surface. Then I noticed that when I neglected oiling the boots, the leather parts would quickly turn dark as water/snowmelt saturated the outer surface, and that was when my feet felt the wettest. I came up with the conclusion that even if the boot's materials are not letting water in, if there is enough evaporation happening on the outer surface, it will suck enough heat out the interior of the boot, tricking me into thinking cold water is leaching in.
So I switched to all-leather work boots for winter chores and regularly hot-pack them with snowseal. When ice melts on these boots, it beads up on the surface rather than saturating into the material, and I noticed that my feet never feel wet. A drop of water will evaporate much slower than the same amount of water spread out across a surface.This is one of the many reasons why I am being converted to quality leather footwear with minimal synthetic elements. Aside from the synthetic foot stink, I think all-leather with a membrane liner is a decent combo. Like others said, the membrane is sort of a back-up in case your leather breaks down or forgets to remind you to oil it.
I have also come to realize that it is unreasonable to expect any boot other than rubber boots to be waterproof. Good boots should keep your feet dry in reasonable conditions, but to expect to be able to stand in water and not get wet is not something I do anymore. I think the better option is to keep your feet out of standing water, or wear rubber boots.