Rules and Errata

There are still effects in post-Core cards that increase difficulty (such as Boggard and Werewolf). But if this is considered not enough to validate the power of an adamantine weapon, the sentence: "If a monster has an effect that increases the difficulty of this check, ignore that effect." could be changed.

Keeping with the theme of what adamantine does, the power should be:
"On your combat checks against a monster with this weapon, ignore the monster's resistance to Melee."

But if you want something more in line with Veteran scaling, I suggest this:
"On your combat checks against a Veteran monster with this weapon, add #."

I'm not clear what the actual argument is here.

It's not in question that it works on Werewolf/Boggard powers and Resistant, is it?

If it's more, (d/sh)ould Adamantine work on non-monster (ex: scenario, location, and hour) powers that increase monster combat difficulty #s - sure, seems reasonable. Does that need a change in wording? Maybe. I'd encourage you to include an adamantine weapon or two in Core 2, and get a good wording on it. Also, make at least one of them not a substandard defensive monk weapon.

If it's "Adamantine" should work on Veteran formula like 10+##, then... no, it shouldn't. That would be against the entire premise that let Core get away with putting Veteran on so many cards, and is antithetical to the design principles of the changes in Core+.