Ah, I am aware of the countermeasure- but consider that one suit of Spartan armor is worth a whole Battle Group in production cost; the weapons that are mass-manufactured were designed to combat targets that are not shielded, so of course it would be less effective against a Spartan.
The Spartan-IV program weakens this point, since if EVERY soldier had a Mjolnir suit, it would definitely be reason for a countermeasure... Just one of (many) reasons I disagree with the Spartan-IV's presence in the Halo-verse.
I just want to point something out dealing with the bolded part. I have no idea where you got it in your head that a single suit of MJOLNIR armor costs as much as a UNSC Fleet Battle Group. If that were the case don't you think somebody would have pulled the plug on the project for its expenses?
The reality (with in the lore that is) is that the Spartan-II project had an over all budget which would allow the UNSC to build a single new battle group if they canceled the project. This includes upgrades to the MJOLNIR armor system, maintenance and the building of new suits among other things.