TolTol,
Just to be a pedant....
The necessity for you to disconnect the A/P during a VNAV PTH dive for the profile isn't a problem with VNAV per se, it is a problem with the rate at which the aircraft responds in pitch - placard limit reversion - to an upcoming overspeed situation.
The transfer function time constant is way out in my opinion, which, of course necessitates your timely intervention, one way or the other.
The FMC Guide even mentions this in the section on DES. Update 10.2 shifts the reversion to Vmo/Mmo - 7 kts to try and combat this problem but it is still too late in my opinion.
In my experience, those who mis-trust VNAV computed profiles inevitably don't fully understand what the algorithms do and why a deviation may or may not result during the descent. As for energy compensation, well.....
It should be no surprise that the pilot is responsible for speed control during a VNAV PTH descent. Again, the FMC Guide states this too!
The systemic misunderstandings that underpin BOAC's observations are quite prevalent and really just reflect a laziness in terms of getting to grips with the functionality Boeing have placed at our fingertips.
My $0.02.