I mean no offense to anyone when I say this, so I hope it doesn't get taken as overly harsh and get me crucified by people who feel compelled to defend them…
But the overall truth is just that Rat Labs doesn't really know how to balance these types of items.
The gear they've added - the 1, 300, and 600 weapons and the 1, 250, and 500 gear all follow a very logical, progressive, linear path and they do a good job with these items.
But these legacy items, such as the old helms, don't fit in that gear progression.
And since they don't fit in that progression they would need a defined identity before they can be balanced and they do not have that.
What is the NMH's target identity? I asked Gimli this on channel 4 when he was pondering how to nerf it, and he had no answer.
How can it attempt to be balanced if there's no identity and it's outside the normal progression? It's just throwing darts at a board.
I think it could have had lots of different identities, for example:
a) It could be a Necro-focused wiz hat intended for between Prof and Expert Necro helms.
If this were its identity perhaps it would have:
20 AR
2.0 Weight
+15% Necro SDM
-5% Thaum/Elem SDM
Nourish
Level 350
This would put it at more necro power than a proficient helm, at the cost of dodge and some other SDMs. You get to have level 500 type SDM earlier than normal, at a cost.
b) It could be a Necro-focused hitter hat designed as an alternate path to Prof/Expert Warlord's.
If this were its identity perhaps it would have:
60 AR
6.0 Weight
+10% Necro SDM
+5% MDM
+1 Attack
Level 350
This would give you a necro boost as a hitter, at the expense of some MDM and Nourish from the prof/expert route.
These are just illustrations of how starting with an identity then dictates the direction of the stats. I'm not necessarily suggesting that either of these identities is the "right one", but I guess my overall point is that it feels like it has no identity and that is the underlying issue.
Instead of having an identity it was just "too strong, make it weaker".
I agree that the helm in its previous state was OP. It needed a change. But without an identity all of these items will invariably be "OP" or "not for anyone" rather than at least having a niche audience.