I agree with this. But asking the candidate to invert a binary tree is one thing and asking the candidate to flawlessly solve back to back leetcode "hard" level

I agree with this. But asking the candidate to invert a binary tree is one thing and asking the candidate to flawlessly solve back to back leetcode "hard" level problems is another. I think the latter is mostly what people lament, where you end up doing so much prep that it boils down to "I've seen this one before" and you proceed to regurgitate what you memorized.