The liver itself might be simple but the complexity is in the enzyme pathways. The marker of "elevated liver enzymes" doesn't tell us much.

Taking a step back, there is only one organ I will admit we understand better than the brain, the heart. It's the only organ where we know enough to replace it with a mechanical counterpart. We don't know enough to make it last a long time, but it's enough to prolong someone's life in a meaningful way.