I guess a GM could moderate the likelihood of success,
A CM certainly could. There are also optional rules that let players pick their own operating objectives and dynamically control overall difficulty by empire size too. There are some very easy way to control which missions could be generated but the missions are designed with a mean difficulty in mind.
But really you don't want to, otherwise players will just build war-fleets (or other "specialist" fleets) and wait for the appropriate objectives to roll it. You want the players to build more varied fleets with multiple capabilities (makes resource management more interesting for a start, and rewards intelligent fleet design rather than just building a honkin' big fleet),
I believe it creates a less war driven campaign (which we fully support and has a rightful place) and a real need for all those ships outside a tactical game. Also, fleet deployments become exceedingly important to watch out for pirates, patrol the borders, and react to things like a science station in need of rescue or finding a traitorous Gorn.
Of course, it might be entirely appropriate to adjust probabilities of different types of mission appearing according to race (you would expect the Federation to pursue more science-related missions than say the Klingons for example - although the Klingons should still expect to see some science missions),
That is also a possibility I could work in. Right now everyone pretty much has an equal shot at any mission type showing up. My reasoning there was that even if a race is more warlike it still needs to drop off medical supplies or investigate space relics.