It definitely seemed most common among Grunts, yep - probably because Sentries I usually found alone and tended to kill in 1-2 volleys.
I used Thermal weapons, but didn't focus on them. My usual loadout was a pair of Kinetic weapons and a pair of energy-heavy weapons - EM or Thermal made very little difference to me. I've experimented a little bit with Kinetic-heavy or EM/Thermal-heavy loadouts in Alpha 3, btw, and while they kinda work, I'm still finding the 50-50 split loadout to be best for me. (when I get a 5th slot it's just whatever my heat dissipation will allow and what I have more available of)
I was playing a lot of the run well past my bedtime, and therefore had much of my attention on keeping my play tight and not making stupid mistakes due to tiredness, but both situations were occurring. Sometimes it was the guy I'd be having a multi-volley shootout with, but I remember at least a few times when it was a bot in the back that I hadn't touched.
I'd tweak the thresholds to make meltdowns less common. In fact, in my opinion, you might consider making it impossible or near-impossible unless you a) transfer excessive amounts of heat via Thermal weapons, b) shoot off enemy heat dissipation utilities, or c) the robot's core is brought below 50%, at which point it disengages the safeties. Frankly, seeing the opposition fall apart for no clear reason was welcome but a tad anti-climactic, and made them appear shoddily made, which doesn't fit with how I envision the facility and its robot guards.
