(this review wrote by Japanese who don't speak English) My first MAL review. Almost Japanese think best Ninja comic is Naruto. I think, Naruto is very good until the Ninja test(Chuunin Shiken). these days, Japanese Otaku thinks best ninja fiction is Ninja Slayer. novel is great, comic is not bad, but, the anime is shit.and, Under Ninja is? this comic is very good.
Under Ninja's ninjas use cool gadgets(optical camouflage, satellite weapon) in real world.
Japan is safe country, and no one use gun, so almost scenes normal Japanese(whore, dull middle-aged businessman, boy who doesn't to school, or more strangers) move and talk.

There are two side comicality in this comic, Normal Japanese strangers talks and living, and, Real ninja battles.

I think this comic's ninja is like a Iron Man or Batman.
They are special people and use real comical gadget, and they are not immortal in real world.
so, I think Foreign Nerds love this comic.