The main problem of polarizers on ultawide lenses is an uneven rendering of the sky. This is because the polarization of the sky is not uniform. It's most polarized at 90 degrees to the position of the sun. So a very wide view of the sky with a polarizer may make it a deep blue in one part of the image (wher it is most strongly polarized) and pale blue in others (where it's weakly polarized)..

If the sky isn't in the image then there is generally no problem. It certainly will not cause distortion (assuming you buy a decent polarizer and not one costing $5 from China via eBay!). It really won't cause any color shifts either other than increasing saturation of some colors (which is presumably why you are using it anyway). Some polarizers on some ultrawide lenses might cause minor vignetting in the corners of the image, but that's lens and polarizer specific. There are "ultra slim" polarizers designed to minimize this effect, but they are more expensive and not required with most lenses.

The best polarizers are probably the B+W ones like this - B+W 77mm Circular Polarizer with Multi-Resistant Coating

The milticoated Hoya is also excellent - Hoya 77mm HD  8-layer Multi-Coated Circular Polarizer

Less expensive, but still good and still multicoated are the Hoya Alpha line of filters like this one - Hoya 77mm Alpha Circular Polarizer

It's not worth buying really cheap polarizers. They tend to be uncoated and some can also soften the image.

It's hard to give definitive limits because it depends on the part of the sky you are looking at and the degree of polarization you dial in and is influenced by things like the presence of clouds and large bodies of water which reflect sunlight. It also depends on the time of day and a bunch of other factors (see http://en.wikipedia.org/wiki/Rayleigh_sky_model). So you can't say that using a lens with a field of view greater than X degrees is a problem because X isn't constant. However the wider the lens is, the more likely it is that uneven sky polarization can be an issue.

It's not something to worry about too much, but it's something to be aware of and to watch for when shooting. Polarizers are useful for things other than just darkening the sky, so a polarizer is still very useful on an ultra wide lens. They can reduce glare from water, saturate colors and even act as an ND filter if you need longer exposure times (e.g. when photographing flowing water).