Thanks for the kind comments!!!
This method has a few drawbacks, so it is still better to use high quality ND filters.
1. When someone looks at the Exif, they will see a mismatch. The image will look like a long exposure photo, but the shutter speed value will not.
2. The shutter speed will be much slower than with ND filters (there are intervals between shots and there can be many shots). This means, that small moving objects are "more critical" when shooting this way. So not everything can be shot like that.
But there are some advantages.
1. When you stitch shots together, you improve the clarity of the shot and this dramatically reduces the amount of overexposed areas (for example, highlights on the surface of the water from the sun).
2. If the weather conditions are not favourable (rain, fog, splashing water, etc.), you can save on expensive ND filters.
3. By using this method, it is possible to reproduce very long exposures without the degradation of colour that is possible with "strong" ND filters.
4. "Backup" method when no ND filter is available