It's understandable as to why you'd have trouble landing ftilt. Fsmash is... well, Fsmash, dtilt comes out quicker, Dsmash is much faster than it, etc.
This move is excellent on taller characters. Swipe at their feet and you'll probably stand a decent chance at going right under their shield. This is a nice follow-up if you've already eaten their shield with an Fsmash that might've been improperly spaced (as in, they were closer to Zelda than you wanted, shielded, moved back a bit, and then you ftilt at their feet for a virtually guaranteed shieldstab on a lot of characters). There are some characters that I just won't use this move on when they're grounded. Squirtle and Olimar are a couple of examples. This is because you're probably not shieldstabbing thanks to their height.
Like someone said earlier, angling ftilt upward is a nice answer to short-hops. Other than that, I would suggest against ever using this angle. The standard ftilt works a lot like the lower-angled ftilt, except you're probably not going to hit someone who is shielding with it. Pretty standard move.
Sometimes I'll turn away from my opponent and then turn and ftilt. If I don't want to have them wherever they'll be after ftilt, I Fsmash instead. I got into a bad habit of using Nayru's when someone was behind me; ftilt is probably a better option for this unless you absolutely can get off a better move than ftilt. Like I said earlier, I've hit with it a fair bit after a blocked Fsmash one way or another, and I think that being able to remove the chance of DIing out of the attack is a great reason to use it.
So essentially, my opinion is that you should just start replacing it with Fsmash when you're spaced out nicely for the move, because even though the final hit of Fsmash barely, barely outranges it, that comes out a bit later than ftilt, so why not get that range a bit faster? Angle it downwards and you have a better move. Upwards is good for short-hops. It's situational, but definitely usable, imo.