Even with whiteboard interviews, you still need to prepare for them. I would say I probably spend at least 8 hours preparing for them mainly because of its arbitrary nature. I have no idea what they will be asking.
With a take-home programming challenge, there is less uncertainty. By looking at the exercise, you can make a good estimate on how many hours it would take and see if it's worth doing.
I would prefer it that way too but whiteboards aren't getting replaced with take-home assignments. Instead they're a step towards the whiteboard stage. The whole process is just getting sillier compared to other professions.
Even with whiteboard interviews, you still need to prepare for them. I would say I probably spend at least 8 hours preparing for them mainly because of its arbitrary nature. I have no idea what they will be asking.
With a take-home programming challenge, there is less uncertainty. By looking at the exercise, you can make a good estimate on how many hours it would take and see if it's worth doing.