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.
Parodies that use the scene where Blondi is killed with cyanide.
Parodies which reference the Holocaust or other Nazi atrocities, such as this one, can easily invoke this reaction.
This parody. Needs no real explanation when you watch it.
Parodies that depict Hitler being abusive towards Eva and forcing her to perform sexual acts for him.
Regardless of the parodies' subject matter, some take offense on the videos as they felt that it trivialized the war and were done at the expense of Holocaust victims. This parody, for one, gained some controversy for juxtaposing Hitler and American Girl dolls, although some argued that it was made as a commentary on Mattel's changes to the company and was no different from a Mel Brooks comedy.
In fact, such opposition is acknowledged by the Unterganger community as a recurring issue, and while some would just jokingly say "Welcome to the Internet!", others prefer a reasonable approach and give an explanation over the parodies' intent.
HRP has made at least a few videos trying to explain that the Downfall Parodies do not support Nazism in any way.
Fan-Preferred Couple: Gunsche and Gerda are paired together in several videos, interestingly enough.
"Funny Aneurysm" Moment/Harsher in Hindsight: This parody depicts Hitler's TV channel getting hijacked by Fegelein via a transmission showing Eduard Khil singing Trololo, during which Hitler suffered a stroke and was taken to a hospital. Years later, Khil suffered a stroke that took his life.
Growing the Beard: When it initially took off, most of the videos were just Hitler ranting about whatever he was informed of in the video and only used two scenes from Downfall, Hitler's rant regarding Steiner's failure and the scene where Gunsche informs Hitler of Fegelein's desertion. The videos began to evolve once more Downfall scenes were used and more named characters emerged, being given various personalities rather than just being unnamed informers (Fegelein and his antics, Gunsche as the Captain Obvious informer, Jodl as the Commander Contrarian, etc.)
Kate Bush as well. Not only is she the most talented singer in the Parody Universe, but she's led entire armies against Bieber's fans and several times and always won. I think it's worth pointing out that when the United Bieber Fever Alliance first appeared the entire German army couldn't stop them, but a platoon of Kate Bush fans come onto the scene and they immediately start dropping like flies.
Nightmare Retardant: The Goebbelman in itself is a generally scary video in the series about the ghost of Joseph Goebbels haunting the bunker, possessing people as he go... But if you look closely in the background while his is possessing his wife trying to convince Hitler into going to sleep, you will notice that the actual Joseph Goebbels is present in blink-and-miss moment behind her.
Strawman Has a Point: Usually Hitler is presented as the embodiment of Hatedom during his rants, but other times he actually presents legitimate arguments. Expect lots of comments saying "Never thought I'd agree with Hitler" when that happens.