Wanda's power really isn't complicated if you ignore House of M, where Bendis got her powers completely wrong (she was never remotely that powerful before).
Children's Crusade, which brought her back after Bendis had her banned from comics for years, even provided a simple retcon for Bendis's mistake: she had some kind of Phoenix-like force within her (due to Doom's manipulations) which made her do all those bad things, she lost the power, Doom stole it and then he lost it.
The problem is writers and editors don't communicate, or read other people's stories. When Al Ewing wrote Wanda in a fill-in issue, he wrote her as having "probability" powers, which was her power set for 30+ years and, while vague, provides a basic limitation (nothing that is totally impossible, only improbable). Even Bendis knew enough to write her more or less this way when he used her in Avengers vs. X-Men. Most writers don't know the character beyond House of M though, just as many readers don't know the character before House of M.
My feeling is they should just ignore House of M/Disassembled and blame them on Doom or the Phoenix or the weather or something. Bendis basically wrote a new character with new powers and new motivations, because he needed someone to accomplish certain plot goals. From 1964-2004 she was in the Avengers constantly and was not at all a hard character to write, apart from being one of the most consistently likable characters. But Bendis seems to have a thing for taking consistently likable characters and trying to break them (see also Beast).
But the evidence for the character being broken is basically one page of Disassembled, two pages of House of M (she doesn't actually appear very much in either of those events) and a couple bad speeches in Uncanny Avengers. Any time someone other than Bendis or Remender writes her, she's still OK.
It's likely she and Pietro will be changed or Secret Wars'd to make them more like their cinema versions, though, so this may all be moot. But the "Wanda is a terrible person" X-fan mantra annoys me because it assumes that a few pages of bad writing wipe out everything a character was for 40 years, and that's giving writers way too much power. Plot is up to the writers, but if they write someone out of character, fans can still say "that's out of character, it doesn't count."
No, it was Magneto's fault for making her strip for him back when she was in the brotherhood of mutants giving her major daddy issues and also the Avengers fault for accepting her clearly delusional ass relationship with a robot and making fake children.
Nothing fake about them. Vision was desperate for kids so she used magic to get pregnant, then consulted with Dr. Strange about whether this was OK, and he not only said it was OK, he delivered the children himself.
The idea that they were delusions born of her desperation to have children was, again, something Bendis introduced more or less out of nowhere. (Byrne wanted to get rid of the kids so he erased them, but his retcon was that the spell went bad and somehow turned demon souls into babies... which is bad witchcraft, but not delusion.) It's really interesting how Disassembled and House of M are so specifically aimed at new readers that they almost go out of their way to ignore or contradict what came before.
After Stan Lee left they established her power as chaos magic due to her birth and making her a nexus being.
Much more complicated than that, unfortunately.
Stan just had her as having "hex power" that causes bad luck to stuff. It wasn't much more than "what if there was a mutant who was kind of like a witch?"
The next writer wanted something more scientific, so based on a reader letter he changed it to "manipulates probability," which sort of became the basic description of her power for a long time.
But another writer wanted her to get more involved in the supernatural side of Marvel, so he had her learn some magic to help control her mutant power.
Still another writer tried to come up with an explanation for how she could have both mutant and magic ability, and introduced the retcon of her and Pietro being somehow gifted by a magic demon.
Then in the '90s Kurt Busiek wanted to find a way for
all these explanations to be true and simplify her powers at the same time. So he explained that her power, thanks to the chaos-demon, was a mutant ability to manipulate a particular kind of magic energy ("chaos magic") which has the ability to disrupt probability and occasionally do other stuff.
You can see how each of these retcons increases her power potential a bit, but the problem with writing her power goes all the way back to Stan and his very vague definition. "Hex power" can mean a lot of things and usually does.