Plasma guns are one of the few things in setting that are man portable and can reliably kill a space marine. They can also unreliably kill the person using them. If you want a comparison, they're less powerful than a Melta, but have much longer range and can be fired quicker. However, they're nearly as big and heavy, so while they can comfortably explode a guardsman or hollow out a Marine's torso for you to curl up inside, you're gonna have to be aware of the threat coming.
Tau plasma weapons are less powerful, and some of the smaller ones aren't actually much worse than Lasguns in terms of self-inflicted damage. Probably closer to Hellguns or Bolters in stopping power though. Eldar plasma weapons tend to be mounted on vehicles, but they do have pistols. I don't know what's up with Tyranid Bio-plasma.
