I never said it was easier, I just said it was not much harder. :)
Like I said, it is sometimes a bit less readable (tree-walking is a good example of such case), but at the same time, what the code actually does is much clearer. You have no hidden costs, and the code is easier to optimize this way.
You mention a case where you do not fully understand why the recursive solution works, in which case you obviously can't easily write an iterative solution. However, in this case, you are poorly equipped to make any implementation, recursive or iterative.
Like I said, it is sometimes a bit less readable (tree-walking is a good example of such case), but at the same time, what the code actually does is much clearer. You have no hidden costs, and the code is easier to optimize this way.
You mention a case where you do not fully understand why the recursive solution works, in which case you obviously can't easily write an iterative solution. However, in this case, you are poorly equipped to make any implementation, recursive or iterative.