If learning to expose correctly wasn't hard enough, getting around ND filters could be a nightmare if you don't have the correct guidance.
There are several brands and styles of ND filters, and here quality matters because they become the front element of your beloved lenses. Therefore, if you use poor quality ND filters, the light passing through your amazing expensive lenses will be degraded.
I still regret spending some money in my first years of photography on inadequate filters. All I wanted to achieve was that beautiful eerie look of silky clouds and skies, but I only knew that they were achieved via long exposures and some filter screwed in front of the lens.
I bought a circular polarizer, which is intended to be used in other circumstances; they are designed to reduce brightness on highly reflective surfaces not to reduce a substantial amount of light. The filters that are engineered for that are ND filters, and it took me a lot of time to discover that.