There are two big loopholes I see.
1) They can cancel with a phone call. This gives you no proof or protection. They can always say (dare I say, lie?), "I called him and told him not to go there." This could be the basis of a lawsuit.
This lawsuit, hypothetically, could attempt to recover whatever you found on the property. They could move to seize any artifacts, coins, etc. in your possession that you cannot prove you did NOT find on the property. So, that gold coin you got from Grandma before she passed on? They're going to claim you found it on their land and as you have no proof of where you did acquire it, how will you counter their claim?
2) Check the limits of your medical coverage. My guess is that you do not have an all-inclusive, uncapped benefit amount. Besides, they don't really care about your coverage. They just do not want to be held liable. So, just keep it a simple release of liability. (However, many lawyers would advise their clients not to grant releases of liability in general.)
I agree that "I will leave the property in the same, or better condition than it was found/used by me." Is much better wording that discussing "holes".
I also agree that you do not want to use the form unless you NEED the form. Don't complicate things unnecessarily