How about the fact that Erza has gotten stronger, and it hasn't worked on anybody way weaker than Erza even when it would have been mightily beneficial.
I simply don't know enough about dragon ball to comment. So again, I don't care about Goku, and it's not a comparison that really matters. Different authors, different series. The author not making her use an ability is not a real restriction. If anything, it's the author's way of telling us that this ability isn't really a game changer. It's one thing if there are any realistic expactions of Mashima ever making use of macro to beat anyone, but there isn't. We're more likely to see Natsu use his flames of savage emotions to beat a god dragon than ever seeing macro beat someone as strong as current Erza.
The orbs don't house Aldoron's MP. They just seal his movements. White Witch never tried to use white out on any of the orbs or absorb the orb in any way whatsoever. What happened was Happy threw the orb at the White Witch's back, and it reacted badly with her by turning her into a tree. She doesn't have to touch any of the orbs to attempt to cast White Out on Aldoron. Heck, even if the orbs contained Aldoron's power, she doesn't have to touch any of the orbs to cast white out on the orbs either.
It worked on Mercphobia because he was looking for someone to take away his power, and thought White Witch was there to help. Ignia being able to burn is still no excuse. Ignia isn't burning everything all the time. She could still conceivably white him out when he's distracted. Again, the fact that White Witch thought that she needed to do anything other than walk up to Aldoron and simply use white out tells us that she isn't confident that her white out can work on a god dragon. If WW could white out anyone regardless of how strong they are, she wouldn't need a way to weaken Aldoron in the first place.