Unfortunately, even with current questions about PACE, we can't show harms. In fact, the data available from PACE couldn't show much of anything concerning safety, except that nobody died, the authors firmly believed it was safe, and rejected the idea any adverse responses were even related to the trial.
As originally proposed, the trial did have the ability to show harms via a decrease in total activity. This is one of those pesky little details that was dropped without changing the language of claims about safety.
There was also a substantial revision of criteria for adverse responses in midstream. You could then be bedbound for a week after exercise without this being counted. (Anyone with direct experience, chime in here to correct me.)