I wouldn't use it since you're not really getting a free domain but instead you are getting a subdomain of their site - com.nu - If you look closely at the domain you will see that any "free domain" you come up with is actually just gonna be a subdomain of that site.
It's a smart trick they're using, but any "free domain" you come up with there is just the same as getting a subdomain on say weebly or wordpress, but with less domain authority.