Many filters seem to cause vignetting (dark corners) in the range of 28 - 50mm (worst vignetting occurs near 35 mm focal length). Note that SOME (most?) CAMERAS ARE EVEN MORE PRONE TO VIGNETTE if the lens and the CCD are not perfectly aligned. I've used three different D7xx cameras and they were all different. These samples were shot with my old D7 which was clearly the best of them all.

Also note the different color casts of the samples. The ND x8 is the worst in this respect. It's clearly greenish. All samples were shot using fixed WB (sunny) and external flash, so the variations are not created by shabby autoWB algorithms.

All the filters I've tested have 49 mm front threads and I can use the original lens cap with them.

Conlusion: although normal (not slim) filters might be OK, if you are lucky, choose only slim filters UNLESS you can test them on your OWN camera before purchasing them.