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.
Or those who only like Keeper Of The Seven Keys 1 & 2 and Walls of Jericho, and those who like these and almost everything else.
Kiske period. He was either the only thing that ever made the band good and someone whose departure ruined Helloween forever, or the single most overrated vocalist in metal history whose obnoxious oversinging ruined the albums he was on, as well as being responsible for Europower/"flower metal" (Sonata Arctica, Stratovarius, Rhapsody, etc.), the likes of which is despised by a good portion of metal fans.
Critical Research Failure: The music video for "Light the Universe", from Keeper of the Seven Keys Pt. 3: The Legacy, is so determined to live up to the album title and uphold this legacy that it prominently features the titular... um... Seven Keepers of One Key Each?
Critical Backlash: Sure, as Chameleon shows, lighter prog-rock with pop elements is the wrong genre for Helloween. However, if you happen to be into that sort of thing, it's surprisingly not-so-bad for its reputation.
Fanon Discontinuity: Aside from Pink Bubbles Go Ape and Chameleon, many fans choose to dismiss the entire Andi Deris' Era, and claim that the band didn't released anything since Keeper of the Seven Keys, Pt. 2.
Harsher in Hindsight: The lyrics of "A Tale That Wasn't Right" take a new meaning after the death of Ingo.
After being fired from Helloween, Michael Kiske declared that he was tired of heavy metal and that he would never be involved in any other metal album. A few years later, he has a guest appearance doing vocals for Avantasianote where he tried to credit himself as "Ernie" in the liner notes of his first Avantasia appearance, but his voice is so unmistakable that no one was fooled. He doesn't even try to hide it these days., Edguy, Gamma Ray, ("Land of the Free", "Time To Break Free" and "All You Need To Know") Masterplan, ("Heroes") and basically any metal band that will have him.
Kiske was also only offered the vocalist position for Masterplan, something he accepted, but only if he wouldn't have to tour, so, as Grapow and Kusch weren't interested in hiring session singers for tours, they rejected him.
Ditto for Sascha Gerstner, who left Freedom Call saying that he would never be in a Metal band again, yet he joined Helloween a year after his departure.
Pink Cream 69 was the former band of Andi Deris, before joining Helloween. Ten years later, Michael Kiske would start the Place Vendome project (now a full-fledged band called Unisonic) with three (later two) members of Pink Cream 69.
Take the lyrics of "Rise And Fall", from Keeper Of The Seven Keys Pt. 2. That song gets funnier when you remember everything Kiske went through after getting sacked from Helloween.
Ho Yay: Andi Deris & Markus Grosskopf has some moments.
Jumping the Shark: Several moments. When Kai left, after Keeper' 2; when Kiske left, after Chameleon; and after Unarmed.
Mis-blamed: Weikath, for Hansen's quiting and Kiske's firing, despite that now both Hansen and Weikath are in very good terms. He's also misaccused for being the only egotist in the history of the band, despite the fact that Kiske had almost completely taken over the band after Hansen left, and the only way he'd let go was for Weikath to fire him.
Replacement Scrappy: There are a number of fans who still do not accept Andi Deris as the vocalist, despite the fact that it's been over fifteen years and he's been with the band longer than Michael Kiske and Kai Hansen combined! Never mind that there's no chance that Michael Kiske will ever return to Helloween.
Tastes Like Diabetes: "Future World", from Keeper Of The Seven Keys Pt. 1, a song so happy (in fact, it was the reason why Power Metal was seen as "Happy Metal" at some point) that it has it's own hatedom, despite being in one of the band's most acclaimed albums ever.
They Changed It, Now It Sucks: There's a very vocal group of fans who revile the silliness that Large Ham Andi Deris brought into the band. However, Michael Kiske was just as silly live, as the Live in the UK live album proves. And they brought out an album called Pink Bubbles Go Ape with a song called "Heavy Metal Hamsters" on it before he was mercifully booted out.
The reaction to Unarmed was... less than enthusiastic. Bear in mind, again, that said album was non-canonical, as mentioned above.