I think my confidence would be tied to if there were any complications on the monkeys or pigs with going deeper.

My intuition says go as shallow as needed to get the data you need as the deeper you go the more something could go wrong, but as we see here, going shallow also has problems.

I'm assuming they tested different depths on animals, so as long as deeper in the animals didn't specifically cause problems, I think I'd be fine with it as a solution.

Now, if they didn't try these depths during the animal trials, well, that's another matter entirely.