"What if the code was indeed garbage, and it cost a lot of time (super precious asset) and brain power (a precious asset) to evaluate and reject?"

Then do so. If you cannot articulate why it's bad, and suggest ways to improve it, then you do not have any business reviewing the code. Review is just as important as writing the code itself, and if you are unable to give that task the attention it deserves, then delegate it to someone who can.