I think a distinction between underrated and underappreciated is needed. I wouldn't call Nightwish underrated, they have an excellent reputation and anyone that has seen or heard them perform recognizes they are very good. (Look at all the youtube videos of people reacting to Nightwish) So, it's more that their style of music isn't what a lot of people like to listen to (notably in the US) , rather than people thinking Nightwish sucks.
I would say AC/DC is underrated, even though they are extremely popular, because their playing has always been criticized for being simple and not being very good - even though playing their songs CORRECTLY so they sound right, is quite hard. How many novice guitar players choose AC/DC songs to learn specifically because they think they are easy?
I think it's similar to country music. Where in the US country artists especially the more modern ones like Brantley Gilbert, Luke Bryan, Kenny Chesney, etc will sell out huge stadiums. But they usually won't even tour in Europe, and if they did probably would only play small venues.
Sure maybe you've got some fans of American country music in Germany or Austria but I'd doubt it's very much.
I think a lot of 80s rock bands (especially Melodirock) that went over "Big in Japan" were underrated in much of the rest of the world like Scorpions, Mr. Big, Night Ranger and Toto.
Though I think the most underrated band of the last 5 years is either Brother Firetribe. All of their albums are great! Diamond in the Firepit was a great surprise this year, probably in my top 5 albums of 2014.
Bands that fit into the symphonic metal genre such as Nightwish and Within Temptation. I say this because it seems like I can't talk to anybody about these bands without getting a strange look because they've never heard of them.
NIghtwish is HUGE in Scandinavia, and Within Temptation is also a well known band here.
Yeah but I wish these bands would be well known in America. I honestly believe these bands could sell out 10,000 seat arenas over here but they end up playing in small venues.