I did some more 'missions' this morning before work. I find the line timer counts down way too quickly. I had a mission where I was to score a 700 point line, but the line ended almost as soon as I landed my first trick, well under a second. Even jumping again the instant I landed wasn't enough to keep the line going. I eventually got it, but it took far longer than it should have.
I have also noticed that you can do things all the time when roaming, but if you need to do them in missions, it's suddenly a lot harder. I flip in and out of grinds almost every time, yet the mission that was 'teaching' me how to flip in and out took much longer than it should have because it suddenly became much harder to actually land them. The same goes with lines. I have never had a 3x, but I get 2x lines all the time when roaming, but scoring a 700 point 1.5x line took me 20 minutes for the mission.