The other thing that probably bears a mention: is that this issue may be a non-issue for many people, and is only being interpreted as one by assumption. No bed is ever perfectly flat, and the people focusing on it are often looking for it in their first 3d printer, and subsequently highlighting any deviation as critically important. Obviously perfection is ideal, but in most cases the typical flatness deviation is fairly inconsequential.
Given the obvious inaccuracy of peoples measuring techniques, a surface deviation of half a millimetre is a fair bit, but possibly not serious if it's really anything less than that. ironically this is what 9 point auto-leveling is for. ..even if we can't do that on the i3 mega.