I've often been accused of "head-hopping" in my stories and not always being consistent with POV. Also, I got a note from a beta-reader once saying that at some points, it sounded like they were reading a narrator's observations instead of the perspective character's.
Third Person Omniscient (3PO) and Third Person Limited (3PL).
I always write in 3PL; I follow one POV character, the narrator knows and can describe their every thought, feeling, and senses. That is the character I want the reader to identify with. It does create limitations on the narration; my character cannot know what is happening across the world. But that also creates opportunities.
The problem with 3PO is that if the narrator knows everything, the reader expects the narrator to tell them everything that is important. If the narrator hops into the head of the villain, we expect the narrator to tell us the villain's thoughts, including their plans.
In one of my stories, my POV character believes she is an orphan, she was with both of her parents when they died. She is adopted and grows up, and while going about her job, encounters an old man that goes to great lengths to help her. He doesn't ask for anything, he's not interested in her sexually (she tries), etc. She is grateful but puzzled.
That man is her biological father. He knows it. She does not. He is not going to reveal this truth to her, because he promised her mother he never would.
I don't want to tell the audience this fact, I want them to be just as puzzled as my POV character, and experience what she experiences when she figures all this out for herself.
It is very difficult to write a mystery in 3PL. It is much easier in 3PO, and I find it easier, in 3PL, for there to be secrets and intrigue. The most dramatic points in stories are (IMO) the big reveals, when something that did not make sense suddenly makes sense to your POV character. For me those are difficult to write if we cannot keep any secrets from the reader.
Writers create an implicit contract with their readers, about what the narrator is going to tell them about. Specifically, we expect the narrator to tell us all the important things. If the Narrator head hops, then when they are in the head of the villain, they have to tell us their villainous thoughts.
There is some drama to be had (we see this in many movies) when the reader knows the hero is interacting with a villain but the hero doesn't know that. And then of course, the villain is revealed and the hero reacts.
But "omniscient" means "knows everything", and if the narrator is capable of revealing what the villain really thinks, we expect them to do so.
I find it easier to create dramatic moments and surprises and unexpected traps and such with just one POV character.
It is still 3PL if the person being followed changes in each chapter. Call that M3PL; for "Multiple". But there is still that "contract", when we follow the villain in M3PL, we expect to know what they are thinking, and inevitably they must think about their plan, their emotions and feelings toward the heroes, and their imaginings of what will happen to the heroes when they succeed.
The problem with "head-hopping", as well as 3PO an M3PL, is the dilution of reader identification. It is fine to just tell a good story, but many writers (including me) want the audience to identify emotionally with ONE character. I want them to experience her ups and downs, successes and failures.
In 3PO and M3PL, that identification gets diluted, we are not elves hovering over the shoulder of one main character, we hover over the shoulders of many, and then may not identify that strongly with any of them.
Third person omniscient is a totally valid perspective, it’s just out of style at the moment. There is nothing intrinsically wrong about using a narrator that has the viewpoint of the author! Charles Dickens, Jane Austen, and J.R.R Tolkien all wrote primarily/exclusively in the third person omniscient, for instance. Ursula K. LeGuin uses the term “Involved Author” to describe this perspective, rather than omniscient narrator, in her excellent Steering the Craft, which I quite like as a framing for this style.
There are tricks to doing it effectively, just as with 3rd person limited— but you only have to look at, say, Terry Pratchett’s prose to see that commentary and observations that originate with the author can be extremely effective and engaging!