The central problem with kneeling chairs is that it's normal and necessary to subtly shift your sitting position on your chair with some frequency (often without even noticing), and kneeling chairs are not conducive to this since they effectively hold your legs in a fixed position.
Of course, if you're switching between two different styles of chair when you need to, that may well alleviate any problems.
I use a kneeling chair(a Varier Variable Balans[1]) and I change position a lot, actually I think I move more with this chair than with traditional chairs. And I can stretch my legs(one at the time of course) while maintaining the correct posture.
Of course, if you're switching between two different styles of chair when you need to, that may well alleviate any problems.