I'd wager that it all comes down to the risks and consequences of going above 100%. If you look at midoriya, when he uses 100% of One for All, it leaves quite a bit of damages. Wich mean that :
The strength of One For All is not limited by one's body
If your body is able to contain One for All, then nice, you can use it at 100% of power all the time, but if you go over the 100% limit, there's no proof that it won't have an effect on your body.
Let's take the All might vs Noumu fight, if you remember well, after defeating Noumu, All-Might was exhausted, and could'nt gather the strength to throw even one punch. While it is true that all might is wounded, and is time limit is closing in, I think it's safe to assume that going above 100% accelerated his exhaustion.
One for All users are able to control the output of their strength
As shown by Midoriya, it is possible to reduces the output of One for All. That's how Midoriya is able to use it without breaking his body in half : He reduces the power output, wich also reduced the harm to his body.
Now if he is able to reduce the power output, and doing so reduces the harm to his body, then I think it's safe to assume that he can also increase the strength of his output, and doing so would increase the harm on his body.
So, Can One for All grant unlimited power ? Technically yes, but using One for All comes with a price, wich is tied to the power output you use. If you go too far, it could probably kill you.