It's got great def. stats (110/105/95), and enough utility to aptly fill a great niche (Defog, Knock Off) while still being able to deal reasonable damage while being resistant to some (Foul Play, Overcoat, Steel Nerf vs. Dark, Knock Off BP buff). Everything that changed this generation made this bird better. You can have unstoppable hazard removal that can hit back and, perhaps not with overwhelming damage, cripple a team (Toxic / Taunt, Knock off, Defog, Roost) and sit out with the bets of them.
Knock Off Buff: Better
Defog Buff: Better
Overcoat Buff: Better
Steel Nerf: Better
On top of that, CAN run other moves to fill niche, such as Whirlwind for Phazing, Tailwind/U-Turn momentum support, SwagPlay if you're a troll, and all of these can fit some of the other good aspects of Buzz's move pool to keep that main niche filled.
Predictability: Overcoat will be Mandi's main ability, w/ the possibility of Weak Armor making use of fast Roosts, but that's hurt by the abundance of priority right now. Most people will probably running Defog, with Knock off and Roost for utility and damage. You can take advantage of the defog turn, but all options have their worries. Knock off can mess with anybody but megas, Foul Play can really mess up a powerful physical attacker, taunt can stop anything that doesn't want to attack. At this point, the "safest" thing you can put in on this guy is a special attack Mega (Gardevior comes to mind).
Of course, this guy has a bit of the 5mss, as adding Toxic would mean it could punish any switch in at all.
Cons: As a solution to SR, this guy still takes 25% damage on the swich, meaning anyhing that might be able to OHKO after SR is going to be a terrible switch, and its speed is also in the bad, meaning anything you switch into will probably get two attacks on you prior to Defog (and after SR damage to boot). That means you don't want to switch on anything that can 2HKO you at 75% health and anything with taunt. Another good answer to this guy is throwing out some set up during the defog turn (Scoliopede or Ninjask come to mind), and get a sub out on the next turn before she has a chance to taunt you (though this is suseptable to prediction).
Afterthoughts: Overcoat COULD be useful in the now popular doubles to counter Rage Powder users... but the low speed could be an issue in that respect.