I'm not sure why Tom does, but I run out of PTO power when using my stumpgrinder if I take an aggressive bite. )
Having multiple tractors, multiple properties and multiple crops, sometimes it turns out that we are operating a tractor one size down from normal. As an example, we operate a 10' disc mower conditioner which is rated at 80 HP min with a 6 cyl 105 HP Deere normally. With sometimes narrow windows of time or weather that tractor may be on another property so the 82 HP Deere gets used. If the grass is thick, and mine is, that little Deere gets worked hard and bogs at times. Ideally I would not run so close to the rating, but it happens. The tractor never overheats, but it is working hard. Often we round bale with a 95 HP cab tractor but if the wife has to take a duty, she only likes to rake and haul. So I give her the 95 HP to rake so she gets the cab while I sweat it out mowing on an open station. All because the hay has to get done in the time we have. Plus if she has the cab and AC, she makes dinner, not me. We can't just trailer this size of tractor easily so we drive them which takes time. So other tractors get used.
Another example is a 12' rotavator prepping veggie fields. I usually don't use it because it is so big, but there are times that I don't have enough hours in the day, so it goes on the 105 HP Deere so i can make fewer passes. With clay in the soil, that 12' rotavator bogs the engine down a little but I never lose wheel traction. I don't know the rating of that rotavator or the actual tine spool length, but I have to take it through 12' gates at an angle. When I talk about bogging down, I am not talking about stalling, it is just that the engine drops by 200 or 300 rpm then spools back up. Traction is never an issue so I'd say that I exhaust PTO HP more than losing traction.
Some day I will own a stump grinder, I bet that must be nice. Right now I dig around the stumps with a backhoe and then burn on them to get them under the soil level.