Variable ND filters r great, but only issue is image quality and range of variable ND
Almost all VND have one issue that they don't give a clear picture at their max ND rating few cheap give a permanent black X shape on picture so when I was ordering online from a local reseller so I thought the safest way was to get a ND filter with fixed ND value, mine is 10x the darkest brand is cheap but works like a charm but it doesn't mean ND Filters have no issues, cheap brands n even highest priced cheery picked ND filters also some pieces give a color cast but that color cast can be adjusted in post processing (if shot in raw)
CPL is a simple polarizer filter, which stops unnecessary reflections off a surface to get better picture
Over all filters should be chosen carefully as ur adding a glass in front of lens n if its of low quality (price doesn't matter) then it will decrease image quality
There r tons of YouTube videos and tutorials how to use filters, I personally don't like other type of filters except these two because the effect which they create can be added in post n using them adds one more glass in front of a lense which might decrease image quality even more ie sharpness etc
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