Ignore resist and Effective has a different mechanic.
Ignore resist = Well ignores resist.
Effective Mythic = It changes your element to the one that’s effective to your enemy.
1.25 x dmg to your enemy.
The problem is we have enemies with immune affix to their weakness element.
Example
A fire fury with the an affix with ice element.
Even if your damage is * 1.25 your damage becomes 0 because of the immunity of the element.
So a lot prefer ignore resist rather than dealing with immune affix. :<
But I do think it should bypass immune because well it’s a mythic >.>