The one with "No speed limits" doesn't mean that you can go 200 in a 90, it just means that previous speed signs no longer apply. So if you had a speed sign of 70 in a road that typically allows to go 90, you can do 90 again.
The one about cows is no joke. Hitting a cow in a sedan at highway speeds is more or less the same as hitting a brick wall.
Edit: actually, given the choice, take the brick wall, because at least a simple masonry wall will crush and crumble and absorb some of the impact. The cow will just gain a sudden increase in its net velocity at your expense.
Germany has a ton of the no speed limit signs.
But in reality they are just no artificial speed restriction signs, meaning drive the limit of the road type you're on.
The kangaroo one just makes sense, it's only exotic because you're the only place with kangaroos, I bet they don't think twice about the same signs for their local wildlife.