Both suffered from having a female sidekick character forcibly inserted (America Chavez in MoM, Ant Man's daughter in Quantumania). I can't comment on Quantumania because I never watched it, but I did have the displeasure of sitting through MoM on a plane once, and here are the big issues I remember:

1. It jumps Scarlet Witch from the "kind of wavering, but not totally evil" character position she (allegedly, I never saw it, but I heard about it) had at the end of her show to a full-on psycho villain. Apparently, this happened because the original plan was to use her in the movie as an ally while positioning her to be an Avengers villain in the future, but the writer for the movie said he thought it would be much cooler if he used her as a villain in his movie instead. So, not only are you whiplashed by her character choices and development if you didn't happen to watch Wandavision, now you're whiplashed even if you did. Also, she's a cunt. They tried to create moral ambiguity, I think, but there really is none.

2. It's disrespectful to a bunch of characters. Beyond the disrespect inherent in skipping all the character development they should have given Wanda to get her to where the film needed her, the main character is theoretically Dr. Strange. The multiversal cameos include a sequence with Black Bolt, Professor X, and Mr. Fantastic. These guys are all supposed to be brilliant intellectual heavyweights, whose powers are arguably secondary to the fact that they can just plain outsmart, outthink, and out plan almost anyone. Mr Fantastic in particular has basically made his entire comic career out of outmaneuvering cosmic heavyweights despite the fact that his team in general and he personally are relatively weak when it comes to their actual superpowers. In the movie, all of them consistently make dumb decisions and try to fight Wanda in the worst, least effective ways possible. In the case of everyone except Dr. Strange, this leads to humiliating, painful, gruesome, avoidable deaths dished out to fan favorite characters in their very first MCU appearances. There is also plenty of the now-classic "Disney undermining or humiliating their male lead" going on in regards to Dr. Strange's overall intelligence, perception by other people, interpersonal relationships, moral code, and combat effectiveness.

3. The inconsistency of the magic. This is really a problem for all superhero fiction that ever gets into magic, but some of them handle it better than others. Wanda is so powerful, with limits so poorly defined or nonexistent, that she can almost literally wave her hand and do anything to anyone. She uses this ability quite cavalierly to kill Black Bolt, Mr. Fantastic, and hordes of mooks, but fails to employ her powers anywhere near as decisively any time that doing so would actually accomplish her goal.

4. America Chavez, who is ultimately the benefactor of or instigator of or contributor to a lot of the aforementioned disrespect of Dr. Strange. She's also obvious DEI bait of the highest caliber, being a spunky, vaguely hispanic, one-of-a-kind ultra special snowflake orphan who prior to being an orphan had alien lesbian moms. Slapping the name "America" on the ultimate globalist wet dream character is just adding insult to injury. Even if you're an NPC who doesn't pick up on that shit, she still gives "obnoxious kid sidekick/mediocre child actor" vibes. You have to be full-leftist "I actively enjoy seeing the title character emasculated in favor of new blood" type to enjoy what she's doing, and even then it doesn't work as well as it does when they do it to older properties, because unlike the older properties, Marvel's characters are coded as "progressive, current, and good" in the leftist/pop culture framing.

Both suffered from having a female sidekick character forcibly inserted (America Chavez in MoM, Ant Man's daughter in Quantumania). I can't comment on Quantumania because I never watched it, but I did have the displeasure of sitting through MoM on a plane once, and here are the big issues I remember:

1. It jumps Scarlet Witch from the "kind of wavering, but not totally evil" character position she (allegedly, I never saw it, but I heard about it) position she had at the end of her show to a full-on psycho villain. Apparently, this happened because the original plan was to use her in the movie as an ally while positioning her to be an Avengers villain in the future, but the writer for the movie said he thought it would be much cooler if he used her as a villain in his movie instead. So, not only are you whiplashed by her character choices and development if you didn't happen to watch Wandavision, now you're whiplashed even if you did. Also, she's a cunt. They tried to create moral ambiguity, I think, but there really is none.

2. It's disrespectful to a bunch of characters. Beyond the disrespect inherent in skipping all the character development they should have given Wanda to get her to where the film needed her, the main character is theoretically Dr. Strange. The multiversal cameos include a sequence with Black Bolt, Professor X, and Mr. Fantastic. These guys are all supposed to be brilliant intellectual heavyweights, whose powers are arguably secondary to the fact that they can just plain outsmart, outthink, and out plan almost anyone. Mr Fantastic in particular has basically made his entire comic career out of outmaneuvering cosmic heavyweights despite the fact that his team in general and he personally are relatively weak when it comes to their actual superpowers. In the movie, all of them consistently make dumb decisions and try to fight Wanda in the worst, least effective ways possible. In the case of everyone except Dr. Strange, this leads to humiliating, painful, gruesome, avoidable deaths dished out to fan favorite characters in their very first MCU appearances. There is also plenty of the now-classic "Disney undermining or humiliating their male lead" going on in regards to Dr. Strange's overall intelligence, perception by other people, interpersonal relationships, moral code, and combat effectiveness.

3. The inconsistency of the magic. This is really a problem for all superhero fiction that ever gets into magic, but some of them handle it better than others. Wanda is so powerful, with limits so poorly defined or nonexistent, that she can almost literally wave her hand and do anything to anyone. She uses this ability quite cavalierly to kill Black Bolt, Mr. Fantastic, and hordes of mooks, but fails to employ her powers anywhere near as decisively any time that doing so would actually accomplish her goal.

4. America Chavez, who is ultimately the benefactor of or instigator of or contributor to a lot of the aforementioned disrespect of Dr. Strange. She's also obvious DEI bait of the highest caliber, being a spunky, vaguely hispanic, one-of-a-kind ultra special snowflake orphan who prior to being an orphan had alien lesbian moms. Slapping the name "America" on the ultimate globalist wet dream character is just adding insult to injury. Even if you're an NPC who doesn't pick up on that shit, she still gives "obnoxious kid sidekick/mediocre child actor" vibes. You have to be full-leftist "I actively enjoy seeing the title character emasculated in favor of new blood" type to enjoy what she's doing, and even then it doesn't work as well as it does when they do it to older properties, because unlike the older properties, Marvel's characters are coded as "progressive, current, and good" in the leftist/pop culture framing.