The biggest problem I can see with this approach is that to some societies and individuals, dangerous material might be desirable. Knowing that there is danger

The biggest problem I can see with this approach is that to some societies and individuals, dangerous material might be desirable. Knowing that there is dangerous material at the site, future generations might want to disturb the site anyway, for the purpose of getting weapons. The warnings are all about the things buried in the ground making people sick and killing them, which is precisely what people looking for effective weapon-making material would want.