I haven't used the filter that much yet. What I noticed is that the darkening happens very unevenly. This gives the image a strong vignette because it is brighter in the middle than on the outside.

It always depends on the application and expectations. It is quite suitable for testing for beginners and hobby photographers. With some lenses there is vignetting, which you can get out again with image processing (it's just one more step).
However, I wouldn't recommend it for wedding or professional photographers, because the demands on the photos are usually higher and if you have to process 1000 photos additionally because of vignetting, it is worth investing in an expensive ND filter set.

That is true. It is a physical challenge to make it so precise and variable, hence the price of expensive vario. As a hobbyist or beginner, this is acceptable or correctable in post. I bought exactly this one years ago and discovered previously undreamed-of possibilities with ultra-long time. I would hardly have bought a "big stopper" as my first filter and such a blatant stopper is often too much... In the meantime, I shoot almost exclusively with professional plug-in filters for combining/amplifying gradients (not only for water, light trails, cloudy skies, but also for urban photography (30'+ "without people").