Pretty much because of the abuse by ISP and the government everything is moving to full encryption.
All the fancy content filter boxes that large companies used to use are pretty much out of business. You can no longer look inside the packets to see what is really going on. All that is left is the DNS and IP addresses. This only gives a very rough indication. A site can have lots of different content and IP addresses can host multiple servers.
The host based ones are a better but what you are really paying for is all the poor guys that have to surf porn all day
. Actually it is the huge amount of data collection to try to keep track of for example the IP blocks the large VPN providers use or maybe some of the IP addresses certain game sites etc. It is not so much the appliance it is the subscription to the database.
Unless the kids are really young content filter is a waste of time. Even a 10yr old will quickly find how to install vpn software. There are partial vpn built into the chrome browser even.
The only real use of parental controls is things like time limits where all internet access is blocked at certain times. You could I suppose try the block everything and then try to get lists of what is allowed but if you have ever run a script blocker in a browser you will know how massive a issue with all the web sites being accessed behind the main page.