I can't see this getting very far before you run out of steam. First you need to define "better off getting their own router" - if you take that to mean you want IPsec VPN tunnels then that's the end of your search, as none of the custom ISP-provided hardware does that.
If you take it to mean that you want VLAN support and a guest network to keep IoT devices separate, then again the ISP-provided devices will leave you wanting. If it means Wi-Fi support, well it's possible to buy routers that aren't 4x4 802.11ac devices, so at that point you might say that a BT Smart Hub is better.
As far as I know none of the ISP-provided kit supports QoS in the sense that you probably mean it. All the devices that are supplied for "digital voice" will be doing QoS internally to maintain voice call quality, but none of those settings will be exposed to you.
If you wanted to do a test then this hardware is usually available very cheaply used, and a BT Smart Hub 2, TalkTalk Wi-Fi Hub, Sky Broadband Hub and Virgin Media Hub 3 would cover a huge proportion of all the end user broadband connections in the UK.