Here is an example that should illustrate the difference.
Pre Legion damage in the form of dots (Mind Flay, Mind Insanity, Shadow Word: Pain, Vampiric Touch, Devouring Plague)
Tic damage numbers.
100, 100, 100, 100, 100, 100
Post Void Form Revamp
All damage numbers
20, 40, 60, 80, 100, 100
Not exactly like that but you get the idea.
With Void Form, you start really low. Then over time you start coming closer and closer to “par” numbers compared the rest of classes DPS balancing. But by the time you get there, literally minutes have passed. Then anytime you get interrupted prematurely via moving, intermissions, silenced or any CC etc. Your damage gets reset again.
So something like this…
20, 40, 60 (had to move or interrupted etc.) 20, 40, 60.
So what you can see is that with this ramp up model, it takes you a long time to do 100% damage potential and each time you mess up for any reason, you have to start over again from the beginning. Whereas before Void Form, all your damage was 100% damage potential at all times just limited by its duration aka DOT duration. All Void Form damage is limited by its ramp duration which will never go above 100% potential.
There are more / other issues but that gives you a nice and quick glance.