Ken Rockwell said "Thou shalt not use polarizer for wide angle"
but for grads, Lee SW150 System/ Lucroit-Hitech 165mm/ Cokin X-Pro are available while Singh-Ray makes filters for these three.
I think you cut it prematurely... Polarizer only makes uneven sky but not uneven water or other non-metallic objects. If you included a little to no sky in your photo, you can still use polarizer.
I use a polariser a lot on a 16-35IIL...it's fine. Mr Rockwell is making big bold nieve statements again. Yes there's an unevenness...but rotating the polariser can place the dark spot in a neat and compositionally strong place...so what's the problem?