Will those work?
The cheap ones can introduce colour errors - they aren't all that 'neutral' sometimes. Sometimes they are - cheap filters are a bit of a gamble. Filters aren't all that easy to make properly. A ND filter has to pass the colours unchanged and the glass has to be of optical standard so it doesn't introduce errors of its own. In photography, for example, you may have a $2,000 lens but when you stick a $10 filter in front of it, every image you make is being made through a $10 piece of glass. In a lot of applications it doesn't really matter, but in some it does. If accuracy of colour rendition is really important, then it's important that the ND filter is truly neutral. If the ND filter you (or anyone) is using seems to be acceptable and you don't notice a shift in the way colours are reproduced, then it's probably good enough. You might even be able to calibrate out any errors it introduces. The bottom line is that the w1070 is an inexpensive PJ, so I think it's easy to overthink this whole issue of the filters and the quality etc.