Anecdotal: Recently done a tournament during the height of the heatwave over in NA. The conditions were honestly appalling: middle of the day, open field, hard dirt, no shade for anyone, and I'd been running back to back matches for a few days already—so have the players.
For one of the last matches of the day which I was the center, I had three separate head "collision" incidents between players. Pretty much all of them constituted regular mutual collisions exacerbated by both players "rebounding" their heads off the hard ground as they fell (and the collisions themselves probably being a result of heat dizziness). After the third incident in the last minute, I waived stoppage and just called the match as a goalless draw—9 head injuries and no goals, a soccer classic.
Long winded way to say that absolutely, basic logic and the rules of common sense dictate an overly "hard" ground can definitely constitute a safety hazard, but good luck being considered for future games if you need to incident report a cancellation because the pitch was too tough. Maybe extra long hydration breaks and a word to the captains beforehand would suffice for this specific field condition/hazard, because some hazards can't be avoided.
