I think one of the main problem with double NAT arises when setting up port forwarding rules for incoming traffic, or allowing it to be set up automatically, with UPnP. There would, I think (?) need to be a separate set of forwarding rules set up, at each level of NAT. Of course, for those of us who simply refuse to allow any port forwarding rules, and disable UPnP, that is not an issue.
Personally I have always wondered whether double NAT might actually be a reasonably safe way of setting up a guest network, isolated from my own private LAN. The first NAT, closest to the internet, would have a WiFi AP, and allow visitors to access the internet, and to access one another. But my own LAN, buried behind a second NAT, whilst still having access to Internet and all my own devices, would not be easily accessible from the guest LAN.
Most routers do offer guest LAN facilities, but it can be tricky to configure, and easy to mis-configure, and you are dependent upon the router manufacturer’s code working. A second NAT seems to me, a more convincing alternative. I suspect there are good reasons against that suggestion, but don’t know what they are, happy to be educated.