Hmm where did you read about the playseat not being stable.
I'm not keen on the center column that sits between your legs on the PlaySeat, not sure how much it gets in the way. That was the main reason I didn't buy one.... instead, I have a GT Omega. Had it for almost a year now, very solid and really comfortable. You should check them out before buying anything.
Take a look at the various sim-rig reviews on http://www.insidesimracing.tv/ - they've reviewed the PlaySeat and the GT Omega, plus a load of others.