D-tilt, depending on your style, can be both good and bad. For people who generally are on the offensive, this move is truly terrible. For people who generally play defensively and punish, it's very good. Reasons:
Offensive:
This move, thought fast during start up, doesn't necessarily work in combos involving you doing constant barrages of multi-hit moves and low knockback combos because of it's ending lag. What an offensive Pit has to have to keep combos going is the momentum advantage, where they keep the opponent in the defensive or in a shield. This is why people tend to use moves that flow easily or can be spaced away in the air, moves like bair (because it's amazing), fair (spacing tool and you can retreat in the air afterwards if you hit a shield), nair (eats shields and can follow up to uptilt ot shield poke), and rising dair (hard for opponent to punish even when shielding it) while the opponent is on the ground. By using d-tilt while on the offensive while the opponent is on the ground, you are probably going to hit a shield with it and get punished because of d-tilt's low shield push and ending lag since you can't move while using it to space away from punishment and it's hitbox is rather small making it somewhat situational.
Defensive: If you are the one going against someone with momentum advantage, d-tilt out of a shield is pretty good (spotdodges take to long to punish out of if someone is comboing on you because they would shield before you would finish spotdodging since good combo attacks have little ending lag after the hitbox is out generally and if they go through you while spotdodging, turning around to dtilt would take too long and you would probrably miss the dtilt if you buffered one since you have to wait for Pit to fully turn around). It allows for a good set-up for an aerial combo if it connects and could help make the opponent lose their momentum advantage this way.