Recently retired DPE…totally feel for the pilots involved.
The FSDO conducts an annual observation for each DPE but that accomplishes nothing since DPE behavior is calibrated to match the inspector’s expectations. Surprise observations would help, but you run into W&B issues that can derail the checkride - you can’t defuel a 172. Video, with an audio feed? That would be very helpful, but that would require inspector time watching the video, which any ASI can tell you they don’t have. Also, it’s very difficult to evaluate “proper attitude” (see PP ACS for landings), as well as other subtle go/no go task elements by video.
DPE behavior being bad enough to warrant this kind of intervention is rare enough that nothing’s going to change unless the problem gets worse.