Flash XL Base Stability
The base is a solid metal construction, thick material but hollow underneath. Structurally you might worry about that, but it’s sounder than you’d think. I’ve seen chair bases fail two ways, thin metal and multiple points of failure. One chair Josh and I looked into worked well until one whole leg of the 5 point base rotated sideways. A thin aluminum wasn’t enough to handle it. An issue I came across with an older 5 star base was that it seemed sturdy with interlaced structure reinforcing a normally hollow base. Turns out, a differential in surfaces (one leg on tile, the others on carpet) popped off one of the legs. What in theory should have been reinforcing actually created extra points of failure. The hollow underside in conjunction with a sturdy thick metal should dissipate impacts across the entire leg much better.
Flash XL Back Stability
The lower back pillow is one of those features that I imagine the average buyer removes and never looks back. As time goes on and your body starts falling apart, that pillow is a lifesaver. I’ve dealt with a lot of back issues as of late and I absolutely need that pillow, despite preventing me from making use of the backrest. In the event you remove all of the pillows, I will say that the Flash series has the kindest headrest. Too many of the chairs punish your skull if you remove the pillows, leaving a solid piece of plastic to collide with. The flash series has the least obstructed surface area for head collisions. The only improvement for the chair on a whole would probably be breathable PVC leather, otherwise it has everything I want.