Here's an item comparison, one is from the endgame of R2 (Hekawt, requiring only 1 win), and the other is a 3 star R3 item that has a total value of around 24 HAR + needing to beat the secret in the current endgame endless mode.
You trade curse of corruption (on the "worse" item) for Anemia 1 and 10% max health. The are currently no curse of corruption items in R3 so that difference doesn't matter at the moment. Why is this difference so small? I've been looking at Argentum for a while now, but wondering why on earth I would use it over my current offhands Wolfswood Reach or Soulcopper Defender (linked in the first comment).
In both of those matchups I would be trading a heavy amount of melee/projectile or just melee protections - and an impactful amount of speed on the latter AND then also losing out on healing and attack speed, for 25% more damage, which isn't even that needed when most of my R2 melee damage sets have ~100%, that increase is not justified for the huge downsides of this item.
Now the problem comes back again when the R2 debuff is removed, Evening Star is almost objectively better, because I'm assuming that'll be removed before more masterwork stars come out, but even then with the small amount of increase each individual level gives, I'd doubt MW4 Argentum would compete still. (And for what it's worth Evening Star is completely fine balanced in R2 since there's a higher amount of corruption items keeping builds in check)
Either the niche of the item needs to be buffed further and increase the damage upwards of 75-80%, or the nerf to attack speed needs to be removed such that it doesn't inhibit dps capabilities. The other option is to increase the item's defensive stats by removing the anemia, or giving it some other special protections/scaling enchant, but for how high the cost to get this item is, it's sorely disappointing for builds I'd want to use compared to other options in the Ring and before.

