Not OP, just a job seeker who like coding interview.
> Otherwise it is half a day of unpaid labor.
The same true for an interview
> Do you have empirical evidence that it can be completed in the given time slot?
As a candidate once you see the specification and you think it's not worth it you can easily can say no to it.
> Is there a guarantee that an actual person will review the submitted code and provide feedback? AFAIK, most companies won't give candidates any feedback / validation of submitted work
That is why I only willing to do these kind of challenges once I had the first round of technical interview and I know I would like to work there
> Otherwise it is half a day of unpaid labor.
The same true for an interview
> Do you have empirical evidence that it can be completed in the given time slot?
As a candidate once you see the specification and you think it's not worth it you can easily can say no to it.
> Is there a guarantee that an actual person will review the submitted code and provide feedback? AFAIK, most companies won't give candidates any feedback / validation of submitted work
That is why I only willing to do these kind of challenges once I had the first round of technical interview and I know I would like to work there