There is a particular feature in TFTDExtender, alien bleeding, that allows aliens to take Fatal Wounds as well. This not only evens out the playing field significantly, it introduces a few new tactics that would otherwise not be practical to employ.
A shot landing but not killing is no longer as big a deal, because either the alien will die soon or be rendered much less combat effective due to health loss or damage to a significant part of the body.
If you want the best idea of how this affects gameplay, play TFTDExtender with the alien bleeding option turned on and you will notice the difference.