Omniscient is not the same thing as head-hopping. In true omniscient (as opposed to writing what's supposed to be third person limited but includes head-hopping), there's a distinct narrator, separate from the characters, who knows everything, and we're always, to some degree, inside the narrator's head, aware of the narrator, even when we're experiencing the thoughts of the other characters. The narrator is recognizable and usually has a worldview and is a distinct, godlike character on his/her own. There's no breaking of POV, because we're always in the omniscient narrator's pov.
The problem with head-hopping is that it breaks the bond between the main character and the reader and creates an emotional distance between the reader and the characters. Every time you change POV, you're breaking the intensity of the bond with one character to offer up a new character for the reader to care about, except that, like people in real life, readers tend to be a little wary about caring about a new person, especially if the reader was just jerked out of the head/heart of the last person she cared about, and then, just as the reader is settling in to really get to know the second character, she's jerked back again and offered yet another character (or even the first one) again, and this time the reader's going to be even MORE reluctant to settle into the pov, and may well stay distanced for the rest of the book, for fear of being jerked around, and in the end will think something like "Hmm, I never really bonded with any of the characters," and they won't come away with a really good impression of the book.
In some stories, that bonding doesn't matter; in emotional stories, it does.
Terry Pratchett does omniscient, and he does it brilliantly, but his stories aren't particularly emotional. Fast-paced and funny and brilliant and satirical, but not particularlly emotional. His later books have been closer to third limited in many ways, and the reader will get deeply into the head of, say, Sam Vimes, but then, every once in a while, Terry The Author (the godlike narrator), will pop in and do a riff on some issue, and while it's brillliant and funny and probably intentional for comic relief, it definitely pulls the reader back from the building emotion in the scene. Because Pratchett's books, at least the adult ones, aren't meant to be emotional in the way that romance or suspense are; they're meant to be funny, with some socio-political commentary thrown in.
Anyway, if you're going to write omniscient, then run with it, use it for all the benefits it can offer, taking advantage of the reader bond with the strong narrator's voice, but also be aware that it will result in a relatively unemotional story. If you're going to write third limited, then run with it, use it for all the benefits it can offer, taking advantage of the reader bond with the pov character, and you'll have an emotional story and you'll have to do a little extra work to reveal the inner workings of the non-pov characters.
But if you try for both, most times you end up with only the flaws and none of the benefits of either one.
If you've chosen third limited for your story, and you're tempted to switch pov, ask yourself why. Sometimes, it's because the emotions are getting too intense for you, as the writer, to deal with, so it's easier (for you, the writer) to switch to someone else's pov, but that robs the reader of the really good stuff of the story. Other times, it may be because you want to be able to tell the reader what the other character's reaction is to the pov character, but note that "telling" is just what you'd be doing, and it's a lot more effective to show it. You don't need to be in someone's head to know he's scared, angry, thinking the pov character is an idiot or whatever. Show that that's how he's feeling/thinking. And stay in the initial POV, observing those reactions.
There may be times when changing POV within a scene will gain you something that's worth risking the loss of closeness between reader and character, but the default should be to stay inside the scene protagonist's pov, and switch within the scene only if by doing so you will accomplish something that justifies breaking the bond between reader and character. It's got to be something that makes the story better for the reader, too, and not just easier for the author. It's all about the reader's experience.
JD