Only think I know is that fragment size can't be too small, maybe you can argue that a nail is untested and thus can make micro fragments? Curious to know why this wouldn't be allowed
Edit: looks like there is a general ban on "unnecessary suffering or superfluous wounds", so if the nails are strong enough to injure but not kill, they are prohibited.
Btd6 is the most well maintained td game out there, super competitive, and you pay like 1 dollar once and can have literally everything unlocked. You only pay for cheats to use in single player. It is one the best monetization I have seen in a game.
But you do you, I guess