To be fair to compare these technologies, they need to be driven the same way. simply saying that "most people don't do as well as the posted price" doesn't mean much-- if somoene drives with a lead foot, they will get lower FE on any car. However, it is true that the hybrids cars do have a narrower "range" at which they operate at peak fuel efficiency than a lot of other cars, I have never seen it explained that way in these reviews, merely that the EPA estimates are somehow dishonest (even though those at least are measured consistently from one car to another)

The fair way to test it would be to get three cars that are almost identical, one with a hybrid drivetrain, one with a diesel, one with a straight ICE. Drive all three exactly the same way, perhaps even perform multiple tests to emulate "speed-limit" driving, and another test for "lead foot" driving" and perhaps even one for "hypermiler" driving. Compare the results of all three cars for all three different driving habits.

What I would expect to find is that both the hybrid systems and the diesel would have about 20-25% higher MPG consistently, with the diesel faring slightly better than the hybrid for the lead foot driving with lots of accelerations, and the hybrid faring better for more conservative driving. I do believe though that the hybrid car designers should design the car FE so it doesn't plummet like a rock between 65 and 75 miles per hour, other cars aren't hit nearly as badly by this speed differential, and in California 75 is about the median speed on the freeways.

Honestly, what someone needs to do is this:

1) Bond a clean diesel engine with a hybrid assist engine
2) optimize the hybrid assist logic so that it has at least the potential for more human control, such as "hill climbing" modes, "freeway" modes and other settings which would cause the hybrid system to behave differently, optimized for the different situations. If I had more specific control over my assist like this, I'm confident I could improve my mileage by 5mpgs.
3) Also, the hybrid engine should be used for subtle changes in speed, or for maintaining speed with the engine at steady RPMs. I belive smoe have called this "reverse throttle priority," basically call on the hybrid engine for speed changes, and only when that isn't enough to get the job done, rev the engine faster. This way the hybrid pack can be used for saving gas even at steady crusing speeds, rather than just for accelerations, and stop-and-go traffic.

Clean diesel + hybrid = very Fuel Efficient car. Although, to have room for all that, this might not all fit in a compact car body, but imagine having a minivan or pickup with the same mileage as a stock civic hybrid!