I've been having very inconsistent luck with Borg adaptation since April 12th's patch. My usual tactic for taking out a group of borg is to catch as many as I can in a pulsewave shotgun's secondary fire, then finish as many as I can before switching weapons. Sometimes this works, sometimes it doesn't.
Since the patch, I've noticed my shotgun's energy type being adapted to in just that initial shot about 40% of the time. If my understanding is correct about how Borg adaptation works, is that it was changed a few patches ago so that adaptation works on a shot-by-shot basis rather than a hit-by-hit basis.
Even then, if I manage to line up three borg in the crosshairs of a High Density Beam rifle, I get one secondary fire and one primary fire before I'm adapted to.
It's not 100%, but the old adaptation system seems to have shown its ugly head again.