The problem with using polas on very wide lenses isn't confined to the fact that a sky, for instance, will look rather uneven across a wide field of view due to the basic pola effect being the strongest at 90° to the light source (the Sun) where, because a wide covers a wider zone of sky, there's more of it on show in the shot to reveal the different strengths of grading.
If you carefully inspect areas in a pola'd shot such as, for example, a wall that's in shadow, it will look a lot flatter and darker due to the loss of that 'internal' micro contrast. The lens will still have caught the detail, but will lose the contrast in the overall darkness of underexposure and just look more flat.
I went through a phase where I used a pola as much as I could, until I tried it for some model shots where it was a disaster and made suntanned skin look like red brickwork. I hardly ever used it again on anything, though I still keep one in the armoury. It works very nicely if you shoot beaches and want to differentiate colours in the water, but at the same time, if you shoot a river, it can lose you surface information because if lets the lens see right through the surface and you end up seeing the pebbles/rocks and lose the sparkle on the surface.... swings and roundabouts. Great for a specific purpose, but certainly not as a general filter just for the hell of it.
Pols come with their own variable filter factors, but the only way I know to get it right is to bracket.
Rob