How is double NAT bad practically?

Anything that automagically open up holes in your firewall (for instance, as BitTorrent client might use uPNP to get a port opened for itself without direct user intervention) is going to fail, because it can't access the 'outer' NAT.

Otherwise, it's a bit of added latency (not likely to be significant) and you're paying to power two devices instead of one.

I had tried double and triple NAT-ing setups for fun. For most intents and purposes, double NAT-ing doesn't affect simple browsing/mail experiences. (adds less than 1ms of latency)

However, if you want remotely access your home network (or services) from internet. It would be a lot more complicated to set that up. That's the only disadvantage I can think of.

Problems arise mainly because the NAT tables on one device fill up or lose track of a particular connection, this type of configuration will cause issues with peer-to-peer technologies that are unable to effectively trace back the network path, MTU path discovery may not function or break and gaming/media services that use uPnP probably will not work unless reforwarding these services manually. [Source]

Solution

Reconfigure the second, inside router as a layer 2 switch, by disabling its DHCP server.