I have both the MVP2 and VV3 and I agree using the VV3 is easier as you can adjust the wattage both ways and if in voltage mode you can do the same. Not sure why the MVP has a different system.
I actually like the MVP method better, now that I'm used to it. The reason is, that to switch modes, you have to remember which sequence, (hold + and press power or hold - and press power) to use.
On the MVP, just hit P and you are adjusting power and in power Mode. Hit U and you are adjusting voltage and in voltage mode.
This makes it very easy to switch modes when I am using the MVP to dry burn my heads.
If you are someone that frequently switches voltage or wattage, I can see how the VV3 is better.
I know the vv3 and SVD are basically the same.
It depends if the VV3 and SVD came before or after the MVP 2. Maybe the MVP2 was the "first attempt" to add variable wattage and the vv3 and SVD are more "evolved ways", or it could be that people complained about not knowing how to switch modes, so they simplified it in the MVP2 so that you have dedicated buttons for specific adjustments instead of more button combos to worry about.