My concern with that is that a person can simply make a vague proclamation about their power that will allow them to have a much larger berth of abilities and moves than if they had simply had to state out what powers they had.
An example would be something like: He has the ability to telepathically communicate and suggest actions to the person, as well alter emotions, could turn into complete control over a person and sending them into a maniac rage that they cannot do anything to stop. There's better examples, I'm sure, but you get the point.
Also, it is helpful for a mod, and for progression standpoints, if the powers are made as such. The number of rounds you have to wait, the exact effects and criteria of the power, all of that is different for each mod if you simply gave them a description of powers.
If you take the example you gave, and his ability to possess undead creatures, one mod might take that as you can only possess one person, and that you cannot control the character while the creature is possessed, while another might think you can control many, and yourself at the same time. Hurting the living with the necrotic powers could be taken several ways, with varying cooldown and varying strengths.
Balancing is also a bit more difficult, for the reasons above and also because interpreting is more subjective then.
It can be done, I'm sure there's a way or even a hybrid. But it isn't the easiest path the walk as a mod or a player.
All that said, I'll give it a go for you if you'd like
. No reason not to give it a shot.