Again, yes, I know it's slow. not saying it isn't. But lets look at it this way.
Mac and Duck Hunt, if they get hit by it, are just dead. Mac doesn't have the recovery, and in the position it puts you, duck hunt is helpless to avoid puff's chase and is essentially dead. Ike is probably dead as well depending on his % and the location on stage you hit him, and it's not hard to downsmash him to punish a dodged smash. No, you probably aren't going to hit a decent mac with it, and you sure as hell won't hit a good mac with it(you shouldn't really be playing a good mac with puff anyways.)
Rosalina, Pit, and Dark Pit are just going to get hit away/bounced off the lip as they try and recover, as they're going to have to use their
. It's not as guaranteed as as stopping DH, but they're still put in a bad position for the MU.
Ness you can just hop out there and eat the PK thunder before he hits himself with it. Ryu all you have to do is hit him after he uses his double jump and he probably won't make it back, just need to stop him from using an upward >b after the downsmash.
In fact, aside from ZSS, Zelda, palutena, mewtwo, pikachu, Lucas, and ROB, most of the characters will have a hard time recovering if you know how to approach their recovery with puff.
If you get the read and hit them with a downsmash, you can then followup and get kills as low as 30%. downsmash isn't a move to use for directly killing, it's used to set up gimps and WoP. So again, the problem lies with the speed of the move, which I agree, is way too slow for a move that doesn't directly kill.