Yeah, ftilt is Sheik's best set up for usmash, but because of Luc's dair most Sheik mains are scared as **** to attempt it. I think Sheik can maybe get a couple ftilts at low % to a utilt or something but it's not nearly as frightening as her usmash.
And, Dguy, tippered usmash kills Luc at roughly 92% with DI. Sounds pretty dangerous to me. I can't see how that kills 'late' at all. Luckily Sheik has such a difficult time landing it against Lucario as opposed to others.
Due to her speed, Sheik does a great job at breaking through your defense. Nearly all of her ground moves (if not all of them) are quite a bit faster than yours. Look:
Jab
HIT: 2
Forward Tilt
HIT: 4
Down Smash
HIT: 4, 16, 23
Dash Attack
HIT: 5 (!)
As you can see, Sheik will have little difficulty beating you to the punch at close range. Her aerials while lacking range (except bair) are all extremely fast as well - nair is 2 frames and bair is 3. Like all matchups, it's important here for Luc to space well and outrange her. As long as you play smart and don't get usmashed, Luc will thrive off the Aura boost and shut her down.
A lot of Sheik players like to compensate her lack of killing power with gimps. Luc's fair, dair, and AS should be able to ward off any attempts at that. Try to avoid using upb to recover whenever you can.
Sheik can very easily play hit and run tactics, build up enough damage, and switch to Zelda for the KO. If both Sheik and Zelda are used, I'm tempted to say the matchup is 50-50. Using either character alone would be around 60-40 with Zelda fairing slightly better against Luc.