So I've been using Mandibuzz quite a bit now on various teams and finding it to be either be a useful niche, or a waste of a teamslot which has several times left me wishing I had another team member in place of it.
Its hard to run Mandibuzz alongside Ho-oh, Lugia and other stealth rock weak mons since it also carries its own rocks weakness and the damage really begins to rack up thanks to the infamous phasing moves Dragon Tail and Roar which are seen on almost every Ubers team regardless of whether its attempting to play stall.
Mandibuzz also seems to suffer from 4 moveslot syndrome since if you choose to run Foul Play as a way to answer direct offensive switch ins such as Ho-oh, Genesect (Somewhat), Rayquaza, Zekrom, and just have the ability to deal direct damage, (Brave Bird is weak and apparently so is Punishment even after the foe accumulates a few boosts.) you must forgo either Substitute, Toxic or Whirlwind. (Losing Roost really isn't an option).
Something that really lets Mandibuzz down is its middling speed which allows it be burned or poisoned by Pokemon such as Giratina, Lugia and Gliscor which real wears down on Mandibuzz especially since tis designed to stall. It also means it can't stall out Ice Beams like Lugia can since it won't be able to Roost fast enough. And running Speed Evs isn't worth losing the bulk.
Forgoing Taunt means it can become set up fodder for Spikers and Rapid Spin similar to Lugia and then it gets almost the same moves as the Great Wall, whilst not being not so great. And access to Taunt is one of the reasons to use it in Ubers, since users with a potentially long life span can become hard to come by.
Since I'm comparing it to Lugia I should probably state now that it is not the Great Wall. Lugia can take strong hits from the Physical side thanks to Reflect and greater Physical bulk and obviously Lugia's Special Bulk is phenomenal. It also can't stall out Scared Fire's or Extremespeed. But after testing Multiple sets with move sets combining the aforementioned moves. I've found the set the
Donkey had been using to be the most useful. Which is essentially making it a Mini Lugia with the ability to take on Arceus-Ghost and Darkrai.
Unfortunately its Taunt is quite slow and cannot really Pressure those it taunts besides doing ok damage to Giratina with Foul Play, plus players wouldn't bring in Ferrothorn on Mandibuzz if they knew it had Taunt. And if you attempt to switch in Mandibuzz on any of the common defensive pokemon like Giratina, Ferrothorn, Skarmory, Chansey, and supporters like Forretress, Deoxys Defence/Speed, Tentacruel, theres a high chance they'll have achieved what they needed to do through either setting up hazards, Toxicing Mandibuzz, Spinning on it. And if Mandibuzz loses hazard Support its going to find it very hard to deal damage against quite a few Pokemon since unless its attempting to defeat a specialised threat it should be racking up damage by forcing something to be phased out.
When talking about how Mandibuzz has the ability to take on Arceus Ghost and Darkrai one on one while it is a cool ability it has when you start to think like that its almost as if you're thinking of building a team around it which isn't going to very useful. Unless its been made clear you're going for a full stall team ready to take on all threats in Ubers, such as in one of the great replays Trickroom posted.
To beat Darkrai a scarfer( gogogo Genesect), Lum Berry holder or Sleep Talker generally does the trick. Unless it spells doom for itself by missing Dark Void. And Arceus Ghost is whittled away by being forced into Hazards (either by phasing or attempting to block a spin) and usually doesn't end up becoming too much of a problem unless you make poor plays allowing it to set up a few times. And I mean, if you really want a niche Arceus-Ghost counter Meloetta can be pretty nice!
Although it seems I've been badmouthing the dark bird It does have some great and surprising bulk behind it! Being able to take powerful Draco Meteors unresisted, and most walling the likes of Palkia, Dialga, Reshiram, without their preferred weather and taking weather boosted moves to an extent it certainly can be a bulky bird! The ability overcoat also makes it nice on sand teams since it can avoid the negative affects of it which gives another tool for dealing damage and staying on top against Pokemon which don't find it to be a favourable weather. And due to the montrous attack stats roaming Ubers, Foul Play can certaintly be a coveted move by defensive Pokemon doing a number on the titans, as well as Dark being an excellent offensive type in Ubers.
I'll try to add of replays of good matches using Mandibuzz later, and start laddering properly on the account I signed up for.
Mandibuzz doing some good work.
http://www.pokemonshowdown.com/replay/ubers8110458
http://www.pokemonshowdown.com/replay/ubers8116330