ROB's uair is more of a caught-you-slippin move at higher %s while diddy's is a kill % combo. That's the fear difference.
Aerials
Dair:
-Use to stall your recovery.
-use to follow up a footstool
-use to eat through air dodges.
-Use to guide your recovery (ROB dairs diagonally, so have your back towards the stage to do this.)
Bair:
-If you are against an opponent who air dodges a lot, then throw out a slow move like this one that won't come out until their airdodge is over.
-If you do a short hop bair and DO NOT fast fall (FF) it, then you will auto cancel your landing.
-Packs a punch, can kill off stage at 70+~
-Influences your recovery.
-I'm way too used to the Brawl bair, so I'm still on the fence about this move. I don't use it much yet.
Fair:
-Combos into itself.
-Anti-air approach.
-Out of shield option.
-Quick offensive item grabbing option.
-Gimps well.
-Nearly no landing lag.
Nair:
-Covers rolls.
-Virtually no landing lag
-You can immediately act out of landing this move on the ground. Even if you land it during your spinning animation. You can throw out an input during your Interruptable As Soon As (IASA) frames. Try it out with landing a nair and then jab (great option).
-Nair to roll, down tilt, other quick moves are also good. Basically nair to neutrals.
-Kind of eats through air dodges.
-Good item pick up move as it covers all around you.
-Best offensive landing option 9 times out of 10.
Uair:
-Juggles like crazy.
-Can land with it and lead into utilt
-Can combo out of dthrow and ultilt (me thinks).
-Even when you downthrow and you can't combo with it, wait for an air dodge and then attack with it. So do an empty jump, spook them into air dodging, then punish. I've killed at 60+ doing this.
-Good anti-gimping option as you're recovering.
-Safe sharking method under high platforms.
-Packs a punch.
I'm sure there's plenty more to add, but I hope this helps.