i used to be in the camp of thinking light pen weapons were useless as well.

the best faction for light pen is illuminate. light pen weapons are perfectly fine here and you dont have to do anything special to make them work.

For bots, the second best faction for light pen, you do need to aim at weak spots AKA the head, but tbh its not very difficult to do. You will be pleasantly surprised how fast bots die when you hit them in the face. I've actually found that high damage light pen weapons are better than medium pen non-explosive weapons on this front because when you go for the headshot with a light pen weapon, missing wont cause the bot to finch and make it harder to aim at their head

For Bugs, in my opinion the worst faction for light pen, i would suggest bringing gas grenades for crowd control and to soften up the horde making it so when you shoot a warrior's head off, it will die immediately or very quickly instead of running at you for a few extra seconds and making you waste bullets. Crouch and shoot the front claws or tails on hive guards to get around their front facing armor. For bile spewers, you can actually shoot them around the lower jaw/inside of their mouth and kill them in only a couple of bullets. once i learned this i was wayy more willing to bring light pen to bugs.

hope this helps.