The red ones (bio-stun) are crap. Take about twice as many shots to stun as the blues (electric), unless you get lucky. And if the enemy has armour, it's even worse. I just shot three full clips into a Snakeman before he went down. For comparison, Snakemen take about one clip of 'blues' and 1-2 TGL blunts or 2-3 regular blunts.
Green (acid) are very situational, but not totally useless. But pulse weapons are much better for what they do, although available somewhat later. But you don't really need armour stripping or chem damage, well, ever.
I am only using these for underwater operations. So I have no idea how they'd fair on land. Nor do I see why you would do it when at the same time you also get stuff like proper assault rifles with full auto fire at the same time. Hell, by the time you have these you will have access to Black Ops weapons and Tritanium ammo for use on land. And by the time you get pulse weapons you should have a decent supply of captured alien underwater rifles as well. At least I do. So I am not sure why you'd be comparing those to these.
Gas grenades? The green ones that do basically incendiary damage x2, 10-20 a pop at best, ~half stun, and tend to outright kill people?
Yes. The stuff that instant kills most humanoid enemies at the time of introduction and can still pose a serious threat to aliens even much later on. And they even stun people on occasion as an added bonus. They only really fail when faced with heavy armor like MIB power armor.
They are just a complete cheat code if you get them early enough. Like, crab people raids turn from nearly impossible to one shot kills with those things. And cult missions become a lot more fun when you can bombard your enemies with instant death grenades.
I use them as my primary offensive hand grenade up until I get access to plasma. If the MGL had gas grenades I would literally just arm half of my entire team with them.
KO grenades are what it's at, they are just as good if not better than dart rifle electric rounds in most circumstances, and available from the exact same line of research. When they aren't (because they rely on the 4x damage multiplier a lot), chances are, you want to break out the big guns like Taser Cannons or TGL blunts anyway.
I am unable to find any weapon by that name in the game or in the wiki. What is the actual name of those?
This said, from what I remember taser cannons have really bad range.
Blunts are most likely available before darts, even. They do have some downsides, but I'm partial to TGLs and KO grenades for tougher and softer enemies, respectively... Or I would be, if I hadn't nerfed all grenade ranges and made the red dart rifle ammo actually useful myself (no randomised stun damage, armour piercing comparable to sniper rifles). Now these dart rifles were more or less the same as the new 'blue' darts, and the old upgraded dart ammo with my modifications was even better (that one could regularly one-shot).
I can not comment on modded performance. I will however note that I play the mod that lets you start 1 year early so that might skew my experience.
I do have to concur that underwater is where darts are really better than almost anything else.
They are purely an underwater weapon as far as the description goes and thus my evaluation is concerned. The fact you can also use them on land is just a situational bonus. But really, there is no reason not to pack your transport with weapons for different situations and switch them out depending on the need. And on land you will always have better options.