I usually purchase my .nu domains from a hosting company which has a reseller enom account ($54/per .nu domain).

I was thinking...what if this host were to close down (not that it will but maybe in the future?). I think two people own this hosting business but if it were to close down, this means that I won't be able to renew my nu domain? just because it's on an enom reseller? :/

Would it be best to register this domain at a proper domain registrar?

Reseller's are fine, just make sure that you are the Owner and Admin contact for the domain and be sure that you have a valid email address as that is normally how you would receive contact regarding the domain name. Also make sure that you have the ability to lock and unlock the domain through the domain management panel that you use. In addition to the fact that if they were to go out of business you would be able to transfer it to another registrar. Hope this helps.

I must agree with LaurieJ.

Shorter: if you have FULL control over your domain at eNom, everything should be fine.

But I there is no locking/unlocking function, which is the problem :/

If your host handles your domain registrations, and you essentially have no direct control over them, I recommend transferring the domain to either a registrar or a reputable reseller.

The OP is talking about a ccTLD domain (.nu), not about a .com. ccTLDs follow their own rules; GTLD practices such as locking domains don't necessarily apply to ccTLD domains.

My suggestion to the OP would be to read up on .nu rules on the .nu registry's web site. - www.nic.nu