I'm a big fan of polarising filters for photography, but I was wondering if there are any downsides when it comes to their use with 4K recording on an FS5.

Any opinions?

I can't speak to anything specific to 4K or the FS5 but polarizers for outdoor video do make the landscaping pop in addition to the skies ... if you are shooting WYSIWYG of course. Filter quality matters. Ergonomically, some circular polarizers have a registration mark to help repeat-ability (given the same angle to the sun). On my F3, when shooting indoors with strong sunlight, I find they are a handy one stop ND to facilitate a wider aperture without resorting to the stronger built-in ND.

Well, it is very tempting (and easy) to overuse a polarizer for video. I have been guilty of it myself many times with skies that were just too dark or not uniformly affected from side to side of the frame. Now I rarely use them unless I have plenty of time to setup a shot. Instead I prefer to use Resolve where I have more control to achieve a similar effect in post. A couple of grad filters are still great to have handy, though. No matter what the dynamic range of your camera is, you can still add a couple more stops outdoors with the use of grad filters.

The pola is one of the few filters you can't duplicate in post. It's purpose is to control specular highlights, and when used on talent in a high contrast exterior scene (3/4 backlight for example) where you can't add lights; it can actually give you some extra "fill" by knocking down the kick and allowing you to open up an extra stop or so to pull up the shadow areas to better balance the contrast ratio.
'Twas a valuable tool long before "power windows", etc. in post.
Ken

I'd never use a polarizer on a face. Never. My comments were in relation to landscapes and other outdoor locations that are easy to take care of in Resolve. Of course, reducing reflections on glass, vehicles, etc. are also good times to pull out the polarizer, but even those instances are rare.. Faces? That's what make-up and lighting is for. I'm positive that any face where a polarizer is going to have an effect is going to be an awful shot no matter what.

Doug,
Never say never. :)

"when used on talent in a high contrast exterior scene (3/4 backlight for example) where you can't add lights"

Actual scenario:
Oil platform (Alaska). Camera 60-70 feet from talent. Zoom out from medium shot of talent to reveal as much of surroundings as wide end of lens will allow. Harsh 3/4 back sunlight. Using pola to lose blazing highlight on side of talent's face lowers apparent contrast and lets you open up enough to add that extra "fill" to give agency/client pleasing results in a difficult "no lighting" situation.
A polarizer is just like any other filming tool; it just needs to be used properly and tastefully.

"I'm positive that any face where a polarizer is going to have an effect is going to be an awful shot no matter what."

This old trick has been used on features, documentary TV specials and commercials for years. You just never noticed it because it was so well implemented.
Ken

So going back to the original question --
For shots where you would like to use a polarizer, there really shouldn't be any additional issues to be aware of with the FS5 / 4K.
Even though in recent years I've bought primarily circular polarizers, large sensor cameras are happy with 'normal' linear polarizers as well. (You aren't dealing with dichroics/prisms as in the 2/3' cameras where a linear polarizer would have different impacts on the 3 colors based on its orientation.)
Of course the one thing to be aware of with any external light-reducing filter (pola or ND) is whether the filter reduces the IR transmission as much as it reduces the exposure...

Each cameraperson will probably have favorite uses for polarizers. Mine happen to be somewhat varied (other than the deep blue skies and controlling reflections off water.) I find it often useful when shooting green screen (minimizing backlight shine from painted green floors in wide shots, cutting down on green reflections in news style desktops that are too shiny and reflect too much of the green wall behind the actors.) They're also great when shooting through windshields or when trying to make a tree greener (eliminating the reflective sky reflections from the leaves lets more of the green shine through.)
Trickiest part I've found with faces is that as you minimize reflections along one axis, you are simultaneously maximizing them along the opposite axis. If trying to get rid of top-back reflection on a bald head, it's easy to end up with too distinct an edge highlight on the cheek!