but this also means that this weapon is not really required to win the game though, but if found, makes it easier. In this case, if you need to capture it to manufacture it, the luck factor will be multiplied by you being able to replenish the ammo.
Just in a general perspective, in case of a narrow win/loss situation, not finding this weapon before going to Cydonia might result in a loss, if you find it, it'll be an easy win. So if you keep it very rare and hard to find with a launcher that can't be reproduced, but very powerful and let the player get the ammo for it, there's no way it won't screw the balance. It either will be too easy if found, or too difficult if not found.
Yes, again you make a lot of sense, but I am not sure if this weapon is all that great. It's a variant of the Blaster Launcher with less powerful munitions but a whole clip of them. Destructive? Absolutely. Overpowered? Yes, but I don't know if more than the standard Blaster Launcher.
As it is now, if you don't find it, you still have standard Blaster Launchers - less fancy, but with more oomph and ultimately similar in use. If you find it, you probably get more chances to attack (since it's faster) and can use it when moving more, which is certainly nice, but I don't think it's "easy win".
The situation is exacerbated by the fact that other such super rare weapons are generally of unlimited use.