Here a few arguments against MyNews:
You are any kind of authority? Police, Secret
services, Jurisdiction, Army, Mafia?
The license forbids usage of MyNews by these persons or organisations!
You need a server for more than 1000 newsgroups ?
MyNews is not designed for full Usenet quantities!
You need a server for a lot of customers ?
Because MyNews is not a high speed server - it is a personal server!
You never share anything with others ?
The principle of MyNews is to receive and donate - this is the Internet principle !
(However you can still use the great tools and multi host capabilities)