Backloaded vesting seems like a terrible idea. It puts the employee in a really vulnerable position. As a company grows, the absolute value that a given person generates should decrease relative to everyone else. The function (relative value created this month / stock vested) decreases even more quickly. This may create an incentive to fire (or at least not try to retain) an employee after the first or second years. The founders may not need to increase this person's salary because of the perceived value to come, so this person may end up making less money than later employees.

I would not even think of proposing a backloaded vesting schedule to an employee. There's nothing wrong with an even vesting schedule in terms of employee alignment. If you cannot retain an early employee after year 1 or 2 you have other problems.