From what I understand, unblocked-us is setup by pointing your computer to their DNS server. Their DNS server answers ALL DNS requests with their own servers, thus becoming a proxy/MITM.
E.g. you want to goto Youtube.com. This would resolve to 126.96.36.199
in a regular DNS server. However going through Unblocked-us DNS server
they would yield a different reply, their own IP address. This can be
illustrated better with the nslookup command:
nslookup netflix.com 188.8.131.52 (Google DNS)
nslookup netflix.com 184.108.40.206 (Unblock-us)
This is absolutely dangerous. You are not only querying someone else's DNS server, you are actually proxying everything through their servers as well. Whether you want this or not is up for you to decide.