There seems to be evidence of 9 helicopters lost in the ATACAMS strike. Ukraine used the "pack 1" with cluster munition, limited range, so it rained small bombs all over the airfield. There seem to be satellite confirmation. I have only seen OSINT, but it is speculated that some of the kills were K52 attack helicopters. This is quite a blow to one of the few strategies that the Ruzzis had left.
ATACAMS are nuisance for Russia for sure, and the loss of the helicopters is not totally trivial. But it's also not particularly difficult to adapt to the new threat, and experiencing it in action is good data to feed into the development process.
More interesting is to see where the U.S. sets the ante. These feeds into a calculus about what is 'fair' and what is out-of-bounds when it comes to supplying classes of weapons to proxy forces. In proxy conflicts the more wide-spread opponent presents a much larger 'attack surface' and they have a natural dis-advantage for this reason. And the U.S. has very far-flung bases for protecting their empire.
I don't recall the U.S. (through their proxies) using HIMARS or ATACAMS (or stormshadows) inside of the pre-2014 Russian boarders. Probably the agreement is that comparable Russian equipment supplied to Russia's proxies will not be allowed to attack within formal United States boundaries either. We'll just have to see how things progress, and who makes the mistake of handing modern weapons AND control over said weapons to nutso coke fiends and people of that ilk.
I do not think is a strategy that Ruzzian Army can adapt to. These choppers were as close to the frontline as possible because they need to act with speed and they need to have as much fuel as possible to hoover and move in the frontline. Now they will need to move very far away, so their capability is diminished no matter what "adaptation" they try. Similar, to a lesser extent, with planes.
To note, US did not officially confirmed the delivery, meaning that Ruzzia does not know either if the next week would be the 320 km range ones. Another strategic problem they cannot adapt to.
About what is fair... well, is a war between Ruzzia and Ukraine, so anything military in Ruzzia and the occupied territories is ok and ready to be blown, why not.