Nope, when you register a domain at, for example, Namecheap, this domain is legally yours. It is registered to your name, and even if Namecheap doesn't like you, they can't just take the domain away from you. (excluding for legal reasons, of course)

If they do anyway, you can take legal action and complain at the NIC.

Njalla offers domains by proxy. So they register the domain you would like to have for you and let you use them. However, they have registered the domain in their name, so they own the domain. If Njalla decides tomorrow that you shouldn't use the domain anymore or they want to sell it to someone else, they have the full legal right to do so.

tl;dr Good service if you want an anonymous domain you don't really care about. If you want a domain for something important, don't use them.