I found unlocking the tech tree to be fairly challenging but then the only unmanned probes I've launched were a skydiver to Jool and a lander to Eve. The problem I've been having lately is that the landing legs behave inconsistently for me. I have landers that are stable on the light legs which fall over on the heavy-duty legs. The damping on the heavy-duty legs is fine but it seems like the spring constant is too low; after absorbing the initial impact one or two of them creep down to the fully-compressed position while the opposing legs extend. The spacecraft I've attached these to is symmetric and which leg(s) contract seems to not be deterministic. Again, the same craft balances fine on the light legs.