It's true, Mandibuzz's walling capability is diminished when you consider her SR weakness, relative lack of resists and limited support movepool. But the point still stands that Buzz can make use of what she has very well. Part of the problem stems from when you want to switch something in to deal with Buzz: you want to either cripple her with status, Trick or just some really powerful move. However, Mandibuzz can stop status with Taunt, and her bulk makes it marvelously difficult to bring down otherwise. Direct super effective / boosted blows can wear down Mandibuzz, but most of these users do not appreciate a Foul Play to the face, while slower attackers (Golem, Regice, Eel) risk being outstalled by Buzz. Trick can be a nasty surprise, however either its users can actually force Mandibuzz out (Jynx, Rotoms) or even risk giving themselves away (Now why would you switch a Haunter/Gardevoir into a healthy Mandibuzz?). Like Musharna, Mandibuzz can be such a pain to KO if you do not prepare for it. Am I in no way suggesting it for S-Rank, because of flaws aforementioned, but something as self-sufficient as Buzz IMO deserves better than B, because she can take attacks like you wouldn't believe...
...which brings me to my next points. Firstly, the 'panic button' scenario. What I mean by this is for when an opponent manages to catch you off guard, or sneak a boost right under your nose. You might have to sac your pokemon, but I find Mandibuzz to be able to prevent you from sacking your whole team. For example, a Mandibuzz with some (not even max) SDef investment can survive a +2 Ice Beam from Gorebyss, +2 LO Weather Ball from Victreebel after SR, and even Charizard's Solar Powered Sun boosted Fire Blast or LO Jynx's Ice Beam. This same Mandibuzz can then go on to survive attacks like CB Iron Fist Golurk's Ice Punch after SR, Adamant Zangoose's Facade, Sawk's CB Stone Edge, Samurott's +2 Torrent LO Waterfall, and Scolipede's +2 Swarm Megahorn, then respond in kind to these threats with either Whirlwind or Foul Play. Jesus, that vulture can take some abuse. However, you will have noticed that SR is mentioned only a couple of times, as the others are attacks Buzz would've tanked had she not lost 25% of her health on the switch. Then again, while she can't take these absurdly powerful blows as well if the Rocks are up, but she can still shrug off anything weaker. She also doesn't severely cripple offensive threats like Musharna's T-Wave can, but her STAB (Foul Play) isn't as easy to exploit, since most of its offensive resists (Fighting) have meaty Attack, so it's not necessarily a free switch-in for them. Foul Play also doesn't weaken in the face of Bulk Up/Coil, while a wall who uses other attacks always has to be wary of Calm Mind as well as the aforementioned two. In fact, sweepers that boost their Attack stats only are completely inadvised to do so in order to avoid essentially pwning themselves. My mentions of Eel and Carracosta is to prove how Mandibuzz can hold her own against them. Eel's naturally high Attack makes it susceptible to a 3HKO from Foul Play, while Carracosta can potentially defeat Buzz with LO Stone Edge + Aqua Jet, but a strike from Foul Play can dash its chances for a sweep (more defensive Carracostas can actually be beaten by Toxic + Roost + Foul Play), let's see Golbat do all that? Taunt can even stop them from setting up Coils or Shell Smashes, further making it hard to defeat Mandibuzz.
Reply to Infernis: what makes Mandibuzz different from previous Golurk switch-ins is that a) she can outspeed it with little investment, and thus Roost the damage away to potentially dodge a super effective blow, and 2) she can OHKO if not 2HKO Golurk immediately, the former of which Tangela cannot do without Leaf Storm. Mandibuzz either threatens to KO if Lurk stays in, or Buzz can heal while Golurk retreats so that she can tango with it again. The latter strategy is less reliable than Regenarator, but considering Alomo and Tangela risk being 2HKOed by Lurk on the switch before they can respond, which Mandibuzz has no issue with, makes it quite the check for the ghost robot.
Willing to tone it down to Low-A, but still opting for mid-A.