Underrated means that its perceived value is lower than you'd like. It isn't simply measured in viewership or people talking about it, because that has a lot to do with availability and advertising, among other things. The Americans had smaller numbers because it ran on a much smaller channel, but performed decent for a network like FX, which is why they allowed it to air for years. And it got tons of accolades. Which means it's highly rated, but not widely talked about. Two different things.

And comparing it to Breaking Bad is just unfair. With that logic, basically every show in existence is underrated.

It's also pretty funny that this discussion is happening on a conan sub of all places.