Omniscient narration is where the writer can pop in and out of characters’ heads as they please and it’s one I find can be problematic because with omniscient narration it can be too easy to give too much away too soon. This, I think, can be a particular hazard when starting out writing and appears a lot in early workshops before people start saying ‘am I supposed to know that already?’
I’m not saying that omniscient narration is bad simply hazardous in its own way, like any other form of narration. There is also the added problem that sometimes writers can get a little carried away and change perspective too often. I’ve been editing manuscripts, not only other people’s, where the perspective changes between characters multiple times in one paragraph, and it’s not always a long paragraph. This can be confusing for a reader because it can make it difficult to track with character they’re supposed to be following at that point. This generally proved particularly problematic when people focused on one character for an extended period then had random lines about other characters thoughts. There might be a way to make this work, if you can make it work then I applaud you, but generally my experience suggests that trying to separate the perspective shifts by paragraph at least helps the reader keep track.
However, as I mentioned, another problem with this technique is that switching between characters can lead to there being too much information given away. If I was to change the perspective of my Weekly Serial from first-person to omniscient I’d definitely be tempted to have Bran chime in occasionally when Charlotte is speculating about him and the mystery would be lost (watch out for upcoming reveals). But not to fear, first drafts are first drafts for a reason, so when you’re writing it don’t worry about these things, they can be altered easily enough in redrafting since the advent of computers.
Now I’ve pointed out two risks in omniscient narration I’m going to have to say that I don’t think this means it’s not a functional narrative style, plenty of writers use it very effectively. The most famous example would be Dickens who wrote all his books in omniscient narration and jumped about plenty and people still love them over a hundred years later. The biggest risk of omniscient narration is also the biggest benefit, one thing I miss in first-person narration is the ability to go off and follow other characters to see what they’re doing. In other narrative voices this would generally have to be reported or implied which isn’t always as fun as writing it on the page for the reader. And, as well as risking giving too much away, it can be a good way to play about with information placing it where you want and withholding it subtly by following another character on an interesting adventure. Being able to play with information in such a way casts a bit of doubt on the omniscient narrator being the ultimate reliable narrator. The theory goes that the detach voice of the writer has no reason so lie and that doesn’t mean they have to tell the truth though. Besides, I think that principle forgets that we all have our favourite characters and it can peek through, even when we don’t want it to.
As with all perspectives there are pitfalls to the omniscient but there are a wealth of ways to play with it too and I would encourage any writer to play around with any aspect of writing. If someone hadn’t decided to play around with form and voice then there wouldn’t be literature to begin with, let alone such a vast and rich sea of literature.