I have one of those chairs...the seat stays locked in place fine...unless you unload the seat (partially standup) and pull up on the front edge of it (which relases the tension). When sitting in it it won't slip and is quite comfortable. The advantage of that design is you can fine tune to the exact height you need for long periods of manual guiding your scope in a "somewhat" comfortable mode. The other types seem to be OK also...but I find the idea of fixed stops more limiting as far as exact placement. I never found any problem with unwanted slipping after I figured out the correct way to adjust it..grab it from the back nearest the support bars.
Actually the biggest problem with those chairs...and others...is that some of the mechanism sticks out beyond the sides of the chair so you have to be careful to not catch your foot on it and knock it over.
Bob Berta