I realize I'm way late to this party but I'm using a toon with Def based att and loaded him out with defensive gear and he does very good damage, granted he is an epic toon but I have done the same with uncommon ones too. Maybe the issue has been worked out since this thread was started. One day when I have some extra silver I might swap to atk artifacts and see how much diff it makes, then check it half and half, swapping the weapon to one of each if I go back to Def artifacts, which I'm sure I will, but just to satisfy my curiosity I'll do it and if changing makes a difference I'll update, If I don't update by the end of the year then ATK artifacts were worse. lol Unless someone posts to tell me this has been settled already.

There's nothing to clarify. The details have all been datamined long ago. You can consult sites such as Raid Shadow Legends Tier List | List of Champions by Ranking - AyumiLove to see exactly how each attack scales based on stats.

DEF-based attacks do not benefit one iota from ATK, and vice-versa. The only exception is the very few cases where attacks scale on both stats, and it'll be explicitly mentioned when that's the case.