We do to some extent understand neurons. These systems replicate neurons. So they have very basic "learning" systems.
But would you trust your data set, and decisions on a slug?
For those systems with more power/neurons, we do not know how to teach them (or the "correct/best" maths/weighting/logic for connecting them) correctly. Proof of this, a 3 year old has much more power than these systems, yet would you trust even an educated 3 year old to make medical decisions?