Coincidentally, I just finished reading an otherwise good fanfic that had me ready to grab and shake the author going "for the love of god, please stick to a single POV!" so I'll take a stab at this question by explaining what annoyed me so much about it.
The basic problem with head-hopping is that usually, in a third-person limited POV, the reader can assume that all narration is being filtered through the character's point of view and are their thoughts and feelings even without it being explicitly stated as such. This lets you really get into the POV character's head. Example:
Alex ground her teeth as her friends argued. Why couldn't they ever just go along with what she suggested? Why did things always have to be such a production?
Her hand throbbed with pain. This was taking too long.
In this case, the reader gets to take for granted that it's Alex's hand that hurts, Alex that's annoyed at her friends, and Alex that thinks everything is taking too long.
If you head-hop, you break that contract. The result is that unless you really make the breaks in POV distinct, your reader has to work to assign bits of narration to the characters thinking them. For instance: if I continue the above example by something one of Alex's friends is thinking, the reader has to:
- pick up on the fact that the next part is no longer from Alex's point of view
- figure out at what point the switch happened
- that may mean going back to the "This was taking too long." line and using logical deduction to work out who was thinking it ("right, Alex was the one who hurt her hand last chapter...")
This sort of confusion serves to yank your readers out of the story. The backtracking part is especially bad, you really don't want your readers to have to jump back to the previous sentence trying to reinterpret it. The fanfic I mentioned at the start left me feeling like I had whiplash from the number of times I went "oh, wait, I guess I'm in character X's head now? so who was thinking that before?"
(Note that this is something different from omniscient point of view, where everything is related from the perspective of an omniscient narrator who has access to what all characters are thinking and feeling. For instance, you wouldn't expect standalone lines like "This was taking too long" in third person omniscient unless the omniscient narrator was thinking them!)
Of course, all rules have exceptions. In this case, if you can make it so that the POVs cannot be confused, it's possible to manage it. There's a scene in Terry Pratchett's Nation where two characters from completely different cultures with no language in common meet which mixes their POVs and which I think works very well:
The girl looked at him nervously and said: 'My name is, um... Daphne.' She gave a little cough and added, 'Yes, Daphne.' She pointed to herself and held out her hand.
'Daphne,' she said again, even more loudly. Well, she'd always liked the name.
Mau looked obediently at her hand, but there was nothing to see. So... she was from Daphne? In the Islands the most important thing about you was the name of your clan. He hadn't heard of the place, but they always said that no one knew all the islands. Some of the poorer ones disappeared completely at high tide and the huts were built to float. They would have gone now... so how many were left? Had everyone in the world been washed away?
Due to the totally different perspectives and languages, it's immediately clear for every sentence whether it's Daphne or Mau's point of view, no chance of reader confusion at all.
So in this context, it works, but that sort of thing is... very hard to pull off. Even in the case of Nation the whole novel isn't written like this, it's really just the scenes with the total language barrier and culture shock. In my opinion, this is absolutely a case of "understand the rules before you break them", and in almost all cases you're going to be better off sticking to a single limited POV or writing omniscient than trying to switch out POVs within a scene.