First thing I tried to do as well was mess around with asteroids and got a big smack in the face when i realised how difficult it is to set up anything with asteroids involved. Additionally, I don't know if I am just overlooking things within the mission builder, but there doesn't seem to be a way to confirm two objectives at the same time. I wanted to make a mission where you just grab an asteroid from LKO and bring it back to the surface but the only way for me to do it is to create an objective that says something like "test distance from vessel to Asteroid" but there doesn't seem to be any way to keep checking, so one could easily just get near the asteroid, then leave it and go do the rest of the objectives.
Also, I noticed there is only a slider for setting starting cash, which was an interesting choice seeing that moving the slider one pixel changes the dollar amount you have wildly, and there is no box to just type a number in, which seems to be a huge oversight. Hopefully these things will get hammered out in updates soon, but so far I do think the mission builder is extremely cool as a concept.
Oh, and one more thing, since they didn't have any kind of in-game way to do mission sharing (which I think was a huge miss), where did Squad think people were going to go to share missions? I cannot find anywhere in these forums or in the KSP subreddit anywhere where people are even talking about sharing missions... Just my few thoughts.