The rules don't protect you from confusing your readers. They are guidance, but you can still break the rules and engage your readers. Clarity is not inherent in the rules, but in the ways the author weaves his words and structure. Shifting POVs have been done over and over again without confusing readers; but they've also been done with great confusion.
Point is, there are no rules against shifting POVs, but you have to understand why you're doing it, and how you can use it to engage your readers, instead of confusing them. Simply "following the rules" are not going to help you.
Like someone said, "it depends on how you handle it." What I'm saying is, there are no rules against shifting POVs. And there are so specific rules about clarity and confusion. That's why we are writers, we have to figure this out on our own, and use our beta readers.
I think many new writers are too obsessed with the rules. Sure, you should learn the rules and understand why they're there, but you also need to be flexible -- instead of hanging on the rules like "if I follow them, I'll write a great novel," you have to understand the concepts behind these rules and use those concepts instead. The analogy I often use is "cooking." You can talk about rules like "it has to be baked at 425 degrees for 28.5 minutes" or "it has to be 1 tsp of salt..." or you can try to understand the concepts behind these rules and adapt to your own recipe. If you follow the recipe to the T, you duplicate something. Nothing wrong with that. But if you learn the concepts and adapt, you make new and wonderful things. That's when you become a chef, and not just a cook.