There is merit to the ENY system, but I believe it could go a lot further. I dont think the 12 hour rule is a big thing, the lack of a dynamic ENY scale is.
First, I think HTC needs to seriously re-evaluate the current scale head to toe. There are a lot of aircraft with ENY and OBJ scores that do not fall in line with the ability of the plane (the Ju88 earns FEWER bomber perks than a B17??? yeah, check it out). The P51D has proved to be the horde monkey's plane of choice, which other aircraft can do what the P51D can do as well as it can do it? In short, I believe HTC needs to lower some aircraft to less than 5 ENY (P51D, Spit16, and a few others), and some need a reduction (La7, F4U-1A, and a few others).
Second, I think HTC needs to turn the dial up a bit on the ENY inhibitor. Meaning, make the ENY limiter kick in sooner and make it ramp up that much faster. MAKE the ENY a factor, not an after-thought. It is too easy to just hop in to the top tier planes.