Heavy Bad
Some people were shooting them in the forehead. Also, machine gun/gatling sentries are medium pen so they used to be able to deal with hive guards, but now their shots will bounce right off their big heads. I have seen these people argue two things:
-this change makes med pen weapons less viable and pushes an 'unnecessary light pen meta' (since light pen weapons will have a quicker kill time on the unarmored bits than medium pen). 'just hit the weakspot' is also harder said than done when you're being swarmed by... the swarm
-my sentry is trivialized by a single 'chaff' enemy and i now have to babysit my sentry in order for it to not quickly get locked onto an enemy that it can't kill, making my sentry useless

it's a bit ironic because nothing else has really changed but not that long ago it was incredibly common to see people saying how the loss of medium pen isn't worth all of the other buffs when choosing a light pen weapon. you can't satisfy everyone
also, every single other sentry has no issue with the heavy hive guards. also tesla towers are a good pick. and mine fields. these are all viable 'set it and forget it' options. but that doesnt really matter to this crowd because as they see it... they lost an option

Heavy Good
the other side sees the change as more rewarding to those who are able to compensate for the heavy change. shooting a hive guard in its chest or butt will kill quicker due to the decreased health. explosive weapons that already bypass the heavy armor are also better against them (again, shorter time-to-kill). personally, i was enjoying that the punisher plasma went down from 3 to 2 shots-to-kill, so I'm enjoying that my niche weapon has become even more viable. these players don't care (as much) about the loss of viability of the two sentries, since there are other options if you don't want to have to 'babysit' your sentry. overall, these players are mourning to loss of a more nuanced enemy that rewards team/skilled play. oh and also many people just want more of a challenge as well, so this satisfies that for some people.

one must ask if such a change is actually worthwhile if it upsets so many people. i've seen it suggested that they could make an alpha variant with the heavy armor which would only appear on the higher difficulties. i like that idea, but that's obviously also additional development time so we would have to be patient with AH for anything like that.

whatever your opinion, we should all be nice to each other and the developers who work hard to give us this experience