Let's say you died slowly. You can't mine anything- even sand with a shovel! Also, you can't put a regeneration pot on yourself because you still would wither away. But the pros are: You get speed 4 (I did that because it's like you panicked), fire resistance and extra health for a short period (you probably won't feel yourself for a couple seconds if this happened in real life).

 Here's the con of this potion if you are the one attacking this person or whatever: This potion (hail) can't kill resistance 4+. This is because you can't die from resistance 4- not even take damage. But, nobody can get resistance 4 in survival/adventure without cheats. :)