They have a poor bushing/bearing design and develop play after not very long from what I've seen.
The only bearing in the system is a tiny axial thrust bearing, and doesn't actually provide any lateral support, so once wear starts on the bushings the situation will quickly get worse due to heavily localised (i.e. point) loading on the bushings.
Ideally they need at least one bearing to provide some lateral support and to allow more even loading of the bushing/s, or if they are going to use bushings only they need to be bigger (obviously a hard task in such a thin pedal). These are some of the compromises you make with pedals this thin, and I think other brands have done a better job of dealing with this than HT (particularly vs. the Evo lineup - AExx, MExx, etc).