I had the opportunity to try a bunch of these way back in 1985-6 when they first became trendy. Many then and I think even more now have a poor design which is not sufficiently adjustable to the individual. I suspect this is what makes many of them uncomfortable for some/many users. I actually thought about getting myself one recently and many of those on sale (especially the cheaper ones) appeared to lack adjustments.
They were very good for the back and overall posture. The key issues are that (unless it happens to exactly match your leg dimensions) you have to be able to adjust the following:
final sitting height
leg length from buttock to knee
and probably these angles:
angle of seat
angle of knee/upper calf rest
The problems we found with some of the kneeler chairs in 1986 all stemmed from the fact that you could not adjust them properly for the length of your upper leg and could not get the correct weight distribution between the buttock and lower leg. Try and find one that suits as:
Too much weight on the front of the leg (wrong length or angle) is very uncomfortable and like kneeling across a plank.
Weight on the knee cap (wrong length) rather than the top of the lower leg can be painful
Too little weight on the leg leaves you wobbling about on the top.
