I feel that both the versatility and effectiveness of what role mandibuzz will play is being understated here. I've used Mandibuzz quite a bit over the past couple of days on balance (its best playstyle IMO) and it really does its job well, assuming you don't expect it to more of its job than it should. If played right, Mandibuzz is firstly a good defogger and secondly a wall, and having access to such a fast taunt (80 speed is extraordinary for a pokemon if its bulk and typing) is really amazing for putting a ton of pressure on defensive teams. IMO its best set is Roost / Defog / Taunt / Foul Play, but it's incredibly versatile as a defensive defogger. Honestly, defog isn't even necessary on Mandibuzz, and you could amp up the speed investment and leave off defog if you want to create a purely stallbreaking-oriented set, but I think for the most part defog and roost are absolutely mandatory on every set. This leaves some wiggle room: Foul Play is incredible, but Knock Off can also be used depending if you want to be able to cripple more defensive teams with both Foul Play and Taunt or whether you want to play more of a defensive role against offensive teams, being able to keep up in damage dealt to you by dealing significant amounts of damage to powerful physical attackers. For example, Foul Play has a chance to OHKO SD Mega Abomasnow at +2, whereas mandibuzz only takes ~60% from +2 ice shard with decent physical investment. The next slot is also a tossup, mostly between Taunt / Toxic; because of its versatility, mandibuzz again has an option. Taunt is simply incredible at its speed tier, only needing to invest 44 speed to outspeed 0 speed base 85s (0 speed Suicune, for example.) This means it has a lot of wiggle room for defensive investment after that. Of course, while taunt is very nice it's not necessary, and Toxic can be run to cripple things that like to switch into Mandibuzz, or just to whittle down defensive teams. You can even run Roost/Defog/Taunt/Toxic if you want, but Mandibuzz is going to be helpless against things like SubSD Feraligatr which it should be beating.
Mandibuzz has to take a specific role on your team. It's by no means the best wall in the tier, and will find itself dying due to hazard pressure before it can play a huge role in most games if you try to play it like that. It can take hits from some of the hardest-hitting pokemon in the tier (hydreigon, mienshao, gatr, never ohkod by either nido) but, again, its effectiveness relies on how well your team managing entry hazards. It's safe to assume it will have to switchin on stealth rocks quite often, which whittles down its own health very quickly and leaves it in a helpless position against a lot of common offensive pokemon. However, it is probably still the best defogger currently in the tier. It can defog on almost every defensive pokemon in the tier (the only reason I don't list offensive pokemon in this category is because it will often need to click roost after switching or taking a hit from stuff like Hydreigon, Mienshao or Entei, and this means it can't get off a defog before the opponent can scare it out against offense.) The only SR users that it loses to are Nidoqueen (not counting king, although he fits technically), Mega Aerodactyl, Mega Steelix, Rhyperior, and Mamoswine. You'll notice that two of those are very uncommon (lix and rhyp), and two of those are currently not running SR very often (mamo, you'll only find on HO running lead rocks and aero almost never runs SR.) It's one of the only defoggers in the tier capable of beating SR Krook, which can usually customize itself to beat its defogger of choice, but Mandibuzz's great bulk means even if Krook runs Stone Edge it fails to 2HKO without rocks up first, and even then it has to get two-almost-max rolls assuming it can actually get up rocks in the first place. The premier defogger people usually slap on teams is Crobat due to its ability to beat Heracross and other fighting-types so easily, but Crobat loses to nearly every SR user in the tier while Mandibuzz can beat most of them. To summarize this point, Mandibuzz doesn't find its role on teams for being a supreme wall capable of switching into large amounts of pokemon in the tier, because with a SR-weak pokemon that usually runs mixed bulk investment that role is illogical. Its best niche in UU is the extremely efficent defogging role it can take on, and when paired with its utility and versatility it means it's the optimal choice for a defensive defogger (or just a defogger in general) on almost every balance team. It's going to be an incredible addition to this tier.
In terms of VR rankings, I would estimate it will be either A- or A. It doesn't wall so much of the tier that it deserves an A+ or S ranking, and is generally pressured too much by the hazards it is employed to get rid of. However, it's just too good at the job it has to be a B+ ranked or lower pokemon. It's probably the best defogger in the tier right now and also serves as a great defensive pokemon which can do much more just sit there and roost on offensive pokemon. Foul Play and Taunt allows it to dish out surprisingly large amounts of damage, and the effectiveness of 110 / 95 / 95 bulk cannot be overstated.
tl;dr: great defogger, pretty good defensive pokemon, a- or a, very versatile and effective at its job