I agree. I got my Sim to catch Marc Brandt in Twinbrook.
The difficulty of getting a Sim with the Commitment Issues trait to marry your Sim, without moving them in first, might be relative to the other traits that Sim has. For example, a family-oriented or hopeless romantic or flirty Sim with CI might be easier to marry than say a grumpy or loser Sim with CI. I think, overall, the two big things that matter are the relationship between the two Sims and their moods.
My general game play, if I know the other Sim has CI, is to invite them over in the morning (well-rested) and then run through the romantic interactions until they accept or reject. If they reject, I try once or twice more. If that doesn't work I try to get them into a better mood - feed them, give them a chance to use the bathroom, do some sort of fun activity.
It has been done, can be done, sometimes it just takes a little (or a lot) more work. Good luck.