Due to its "interlock" shape and plastic construction, it does not provide enough support for the stem above it. They deform ever so slightly under heavier loads, which affects preload and causes little play in the headset, which in turns damages the contact surfaces between bearings and cups/races. Using excessive preload to compensate causes the stem to slip into the spacer (it has wider inner part where the interlock is supposed to "lock" in place) and the headset ends up preloaded unevenly, rendering the whole setup useless. These are maybe fine on a road bike, but I had no success with these on my enduro bike. I had to buy metal ones, which work fine.
