About its defensive typing, I'd argue that it's not bad at all, in fact it's pretty damn good for the role Aurumoth plays the most: boosting sweeper.
The last thing a boosting sweeper wants is to be weak to common priority moves (Mach Punch, Bullet Punch and Ice Shard). Not only is Aurumoth not weak to any of those, but it also has a wonderful 4x resistance to Mach Punch, the most widespread one.
With exception of Tornadus-T's Hurricane, it also is not weak to any type common scarf users run (ice, water, dragon and electric - those are relatively "safe" attacking types to be locked into), making it very hard to revenge kill even after a single boost.
It might be SR weak, but boosting sweepers aren't as affected by SR as walls and choice users because they are supposed to switch in once, setup and sweep.
In practice one could pretend Aurumoth had 12.5% less HP while not being SR weak, which means it would still be pretty damn bulky on the physical side.