These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Aborted Arc: In Episode 43, Akira loses his eyesight but manages to rescue his friends in the nick of time. But the blindness doesn't get cured right away, leaving Akira open to be possessed by the disembodied armor of Armor Doggler, transforming him into Underground Knight Unas. Commander Sugata remarks that the onlyway he could have even be susceptible is if Akira was from the Underground Empire. Everyone else is in total shock and it looks like a big reveal is coming up. This arc lasts two more episodes but is then eclipsed by the reveal of Prince Igam being a woman which sets the track for the homestretch of the series. However, Akira is turned back to normal, and the question of is true heritage is never brought up again.
Big Lipped Alligator Moment: The opening credits features a naked body-builder flexing, summoning energy and blasting a big block of ice... which explodes to reveal the five members of the team.
Complete Monster: Emperor Zeba. Tube's dictator, who has absolutely no compunction about killing or punishing his subordinates. His monstrosity is finally revealed in the final episodes where it turns out that he is actually the spawn of Rizal Doggler, a malignant creature who tried to usurp the throne and was defeated by the original Tube royal family. After which he ate his dying father in order to become stronger. Driven by a seething hatred against humans from both sides of the crust, it proceeds to destroy the rest of the original family save Ial and Igam, taking over as emperor and corrupting the peaceful Tube into becoming a militaristic Crapsack World using enslaved underground humans in an attempt to reach the surface world and extend its malicious grip.
Also in Episode 26, the one where Kenta meets Seira and dies in the hands of the Monster of the Week. The sad music did not help.
The Untwist: Prince Igam is actually a woman. Not much of a spoiler, considering "he" was obviously played by a female, with no attempt whatsoever made to disguise the face or voice... and it doesn't help matters much that the title of the episode where this is "revealed" is "Prince Igam, you're a woman!", as though it weren't obvious enough to begin with. Really, The Reveal is the first time you realize it was ever intended to be a secret. However, we do learn much more about her that goes on to be quite relevant to the rest of the series.
To a lesser extent that Takeru's girlfriend is really the princess of Tube, which we find out early in but the Maskmen themselves are ignorant of for most of the show.
Made fantastically weird in the brazilian dub. Igan is still dubbed by a voice actress, but she does not make any effort to make her voice sound a bit more male. Also, some fans are creeped out by the fact that she also voiced Doņa Florinda.
Values Dissonance: The male Maskmen jokingly impersonate criminals to get the female members to strip. That strikes a modern viewer as rather creepy.
Takeru delivering a Get a Hold of Yourself Man slap to Haruka makes perfect sense in context, though that still doesn't make it any less uncomfortable to watch.