Let's not compare the massively OP and combo-able shine to a really laggy F-tilt. Shine can be done in air, done twice in succession, can be jump cancelled, has set knock back protecting from retaliation even at low percents.
U tilt has 6 less frames end lag (15 for F-tilt you're vulnerable, as opposed to 9), and U tilt hits behind, above, in front. In my experiences it's just so much more effective and useful and seems to hit way further than it should. F-Tilt I never get that feeling and I feel exposed.
Against Marth I've also tried F-tilts and it just isn't worth it over all the other moves that could be done instead (after clanking you are left more prone, and Marth can hit faster you at the close recoiled distance than U-tilt). Even if F-tilt connects it sends them off in crappy trajectories leading to no set ups or combo opportunities. Same with using it to edgeguard, D-tilt is so much more superior since it send opponents down and away, and U-tilt pops them up for air juggling.