Hmmm... Here's my take on it since I don't think we really have much info on it yet.
Nobility will have quite a fair bit of power implementing the laws of their land. Reference implicit contract here. These laws will include laws on tax code, criminal code, citizenship, land ownership and inheritance. However, nobility will not be able to make laws that will force players that will do things out of their own will. I once asked the Devs if conscription was possible and the answer was a flat no. So yah, while it is a fair bit of power, it will not be complete.
There is a another part to your question however, that is if they will have complete power over their subjects, which I intepret as being able to enforce their laws. Well, that depends I guess. Some nobles might employ a ton of sheriffs to enforce their laws and some are more laissez faire. Some might even have a "secret police" to enforce it. But whatever it is, I'm sure the Devs will have something in place to prevent the nobles from causing grief, or at least excessive grief to other players. (I am sure they are happy to let psychomaniacal nobles destroy themselves).
Anyway, it's just my opinion and I don't know too much about it, but I hope it helped allay some of your fears.
Best thing you can do is post your question in the next open Q&A or a Q&A on laws or contracts or hope Caspian replies here. 😝