I personally don't think the stack launcher is OP. It is very good in city environments, but the time to kill is long enough that enemies can generally get off a shot or two before they die, if they can see you. It's also not a good choice for missions 2 & 7. In mission 2 you're constantly getting chased by the giant grubs, and I can tell you from experience that the stack launcher doesn't exactly excel in that situation. The reason why it sucks in mission 7 is because you're underground, and there's lots of cover that goes up to the ceiling, which makes the stack launcher incredibly hard to use. It also kinda sucks at killing helicopters while they're flying. However, it is a very good weapon in the other 5 missions, especially if you're stealthy.
The laser is insanely good though. It honestly might be the best weapon in the game. It just melts everything so quickly, and its only downside, being the overheat mechanic, doesn't come into effect in the majority of engagements.
The spiderbot is also pretty good, but the blast radius is surprisingly small. However, the spiderbot makes up for that due to the fact that everything inside the blast radius dies, no matter what it is.
I can tell you from experience that the mines are decent, but mainly because it can be a bit of a pain to lure enemies onto them. They do kill anything that touches them though, so they are pretty good in terms of sheer destructive power.