Having tried most of the lunar/interplanetary probes missions, there is a point that could be, IMHO, improved to make the game more realistic (and maybe even improve the balance).
On most of those missions, you have to use upper stage variations of existing boosters (mostly Atlas/Agena & Atlas/Centaur). Currently, the "Trans Lunar Injection" or "Planetary Injection" steps are under the responsability of the probe safety %. I think it should be more Rocket 60% / Probe 40 %, because if we admit that the probe is managing the guidance/navigational aspects of the step, the rocket upper stage (Agena & Centaur) still have to deliver the burn : if they don't perform perfectly, given the precision required when you aim another planet, the mission is a failure. And the early space unmanned exploration history is full of upper stages failures at this point of the mission (early shutdowns, explosions, under-performance, etc...).
The same applies to the Proton/Zond or Proton/Luna 15 missions : the TLI is performed by the Block-D, which is actually the 4th stage of the Proton rocket*.
* Nowadays there are different Block-D and you have the Breeze-M tug as an alternative.
Edit : Also, in the Sputnik missions, the "Ascent" step should definitively involve the R-7, and at more than 50%
