Random observation of the day: Roy's Dair has 28 frames of landing lag; the same as Ganondorf's. Roy's Dair has the same auto-cancel windows as Marth's before frame 3 and after 55, but it's 7 frames slower on startup to Marth's frame 9, lasts 3 frames less to Marth's 9-13, 5 active frames, has 4 more frames of landing lag to Marth's 24, the sour-spot is 1% or 3% weaker than than the non-spike hitboxes on Marth, and the hitbox is arguably worse since it's just straight down instead of an arc which could have let Roy cover below him in some way. The pros? Roy's sweet-spot and spike hit does 1% more than Marth's and it recovers 12 frames faster. Yeah...
Marth's not a fair comparison considering the hitbox, so let's go to Ike whose Dair is pretty much the same hit-wise. Ike's has the same hit frame, same active frames, and the same 15% sweet-spot. The difference? Ike's does 15% no matter what since he doesn't have a sweet-spot/sour-spot for damage on his Dair, the growth on the regular hit is 10 more to Roy's 90 - not much of a big deal, but it's there -, takes just 3 more frames to recover, but has 5 less frames of landing lag, and auto-cancels before 5 and after 48. I have no evidence for this, but I'm going to assume that Ike's Dair also has more range even if it's slightly, so something like maybe Marth's vs. Lucina's.
If we go purely by landing lag for the frame 16 Dair users, it's, in order of descending, Ganondorf and Roy at 28, Yoshi at 24, Falco and Ike at 23, and Captain Falcon at 21. We can argue if Captain Falcon's Dair is a bit silly - platform don't exist for him -, but that's another story. So, what we can observe not compare is that Roy's Dair has the worse landing lag and the worse auto-cancel windows. Closest to Roy's auto-cancel windows is Yoshi's before 14 and after 50. The after's just 5 frames shy of Roy's, but the before is pretty much phenomenal compared to Roy's; everyone else auto-cancels before 4 or 5 in Ike's case and after ~40: Ganondorf's after is 32, Falco's after is 38, Captain Falcon's after is 39, and Ike's is noted above at after 48. Each other Dair has merit or something "defining": Ganondorf's is just stupidly strong and cannot be teched on ground while having a good auto-cancel window, Captain Falcon's coupled with his speed, maybe a bit too generous hitbox if he uses it while landing, and his infamous speed probably makes his the most reliable while pretty safe considering the 21 frames of landing lag, Falco and Yoshi's have high active frames with differences being that Falco's cannot be teched on the ground, has the most active frames for spiking, 4 to the Capt. and Ganondorf's 3 and Ike and Roy's 2, and his high jump means he doesn't really have to worry if he decides it's a "great" idea to use Dair on-stage... Dair out of hop on the other hand... Anyway, Yoshi's isn't a spike, but can semi-spike because auto-link angle shenanigans and it does a lot of damage if you manage to connect all the hits and with the way shields were changed in 1.1.0, sort of frightening if he hits your shield, and Ike's due to the way he and his moves works, can manage to use his from footstool setups and just compared to Roy's, it's kind of better in all ways outside of Roy having better ground speed and air speed (and air mobility?) to make use of his Dair, but Roy's got much higher landing lag, worse auto-cancels, a weaker sour-spot, and shorter range? that just goes against all the good Roy's got to make his Dair work.
So, we end with this question from this observation: is Roy's Dair under-tuned?