While it is true ROB can force us to approach, we do have the Range advantage, and at higher percents[something like 70%+?] our Projectile has the priority advantage, so if we approach smart, then we're golden.
One thing I always noticed about ROBs, is that if you approach from an angle that is like....okay, picture time:
Okay! If you approach from an upwards angle, from either in front of, or behind, ROB, then Lucario's Range of attacks completely overshadow what ROB can do to stop him, 90% of the time.
In Figure 1, you approach from the front, almost appearing to want to go vertically over ROB. Do not go vertical, because if he times a UTilt or USmash, he'll probably overtake you and start to combo you. So when you're coming in from the angle, you also gain the ability to utilize FAir, as well as a well spaced DAir to land a 'safe' hit and combo starter.
In Figure 2, it is basically the same idea, except because you either approach ROB facing the other direction, or jump over ROB to get into this position, you are using some serious mindgames, and if you space it right you can probably hit more often with this method if you can trick the ROB into using anything but his shield. You'll have to time and FFBAir so you can combo out of it, and at higher percents its a good idea to have an AS handy to fire if ROB goes flying, just to cut off potential escape routes, and follow him with a Grounded ES[Don't even short hop it, staying on the ground when using it has like, 2 frames of lag].
In both figures it IS possible to use UAir as a mix-up, but that takes some more timing and spacing than the other options, but is still very viable.
OH YEA! What if they shield this like a smart cookie? Well, Lucario barely lags on his aerials, so start up some jabs or UTilts, follow into some grabs, and the Combos just start themselves.
The only problem is killing ROB, but if he is the one trying to recover from offstage, we have the advantage, and all it takes is a little prediction or timing[With FSmash if he is going for the ledge, Aura Sphere or other aerials if he is going elsewhere] and we can keep him off the stage without a problem.
Long post? Yes.
tl;dr - I think the matchup is 50:50, if not 55:45 in Lucario's favor if he plays a well spaced offensive game and keeps the pressure.
-DD