Dair is really good, you just need to learn "Hoodoo Voodoo", a skill for technical Foxes who reach LV79 using the "Green Fox" skill tree. It discourages SDI and reduces the number of hits done in some multi-hit moves (dair, uair) with some regularity.
I find dair is good for catching people running away because his legs are falling forward and you don't have to release the stick to create neutral - I feel this helps retain momentum. And for me personally I feel it helps me hold back if I realize I'm gonna miss to cut my forward distance by a lot, which is also useful. It's horrible vs airborne foes but I feel people also play badly in some of the situations they encounter after drilling an airborne foe (that isn't a space animal or Peach).
It's okay vs shield since the stun is basically where you'd put it if you did a completely middle-of-the-jump nair as a pressure mixup. The fact that it hits so low is useful for getting people who WD OOS through you when they expect a SH bair or nair, or FJ anything. I also find people tend to either shield grab at it all the time or never at all for some reason (there's seriously no middle ground) so if you can figure out which one they fall into, the mixup paths and how to influence their shield habits and crap from that point basically plays itself. As long as you're not a bidoof and keep dairing into shine because you want that bubble to melt... instead of shining into grab or straight grabbing >_>
I think dair is generally really good vs crouch because they're holding down to cover or because their expectation is they're going to be hit by a soft move. Most characters who use true-crouch a lot are trying to draw you into doing very specific things or else they're just holding down and hoping the for the best (and the latter isn't too hard to exploit anyway). So if you can spot their lures and how they follow that action up, constructing the plan is simple enough. I think people are generally afraid of grabbing on complacent opponents or nudging them with things like spaced f-tilt, which can be another great tool (f-tilting a non-Armada Peach at like 40% so she counterattacks and whiffs is a reasonable albeit vaguely gimmicky way to procure a free... whatever the hell you want). Push her.
Ledgehop drill gets a lot of hate but it's just used badly, it's not actually that bad.