The biggest drawbacks to Tangrowth are it's typing, Speed, and Special Defence. Because of these three flaws, I don't see why Tangrowth shouldn't stay in B- or drop to C.
Grass typing in this metagame is nothing to write home about. CB Entei, Mega-Pidgeot, Mega-Aerodactyl, Mega-Beedrill, and Heracross are all common in this metagame and will quickly dispatch opposing Grass-types. Even old staples like Nidoqueen and Chandelure don't mind Grass-types. It's biggest advantage is its ability to resist Scald, which Tangrowth doesn't do very well, as it has a paltry Special Attack stat, uses Power Whip to dispatch Suicune or Slowking, and is thus crippled by a potential burn.
252+ Speed Tangrowth barely outspeeds Crocune and Roar variants of Suicune. Considering Tangrowth would be used to break Pokemon such as Suicune, I find it hard to imagine why one would use it over a proven check if you need to sacrifice bulk in order to get the chance to beat the common spreads. If you're slower, you risk getting burned, and can't really beat Suicune anymore. Against offensive Suicune sets, Tangrowth doesn't stand a Snowman's hope in Hell of coming out on top.
This is the biggest deal-breaker for Tangrowth in my opinion. It folds to nearly any Special Attack, prematurely forcing it out or simply removing it from play. This is what distinguishes Tangrowth from Amoonguss. Yes, Tangrowth takes most neutral physical attacks better, but Amoonguss' dual-typing allows it to resist a wider range of attacks, and Amoonguss will always take special attacks better than Tangrowth thanks to its higher special bulk (114 / 80 compared to 100 / 50).
Amoonguss is only ranked in B-, something which I agree with having used it a fair bit over the past week. It's got its upside, but its an extremely passive Pokemon that turns into setup fodder if the opponent is behind a Substitute or if Sleep Clause is already activated (Stun Spore is rude and misses). While Tangrowth does have more offensive upside, I still think that it is held back by being unable to tank hits from a lot of the unboosted metagame. Not to mention that Sleep Powder is significantly less reliable than Spore, making it harder to set Tangrowth up.
A lot of this Tangrowth noise is merely hype from the Iron Chef Teambuilding Competition; players have been restricted into using at least two of ten Pokemon, of which only six are remotely viable. Tangrowth is one of those six. It only works on those teams because they provide heavy support the Regenerator Pokemon.
Well first off there is 0 reason for tan growth to run any speed past speed creeping like blissey or something, as it's main function is as a tank. with max hp max spdef, and either +def or +spdef nature, it can basically take one hit from any mon and do a lot of damage, then switch out and get back 33%
252+ SpA Chandelure Fire Blast vs. 252 HP / 252 SpD Assault Vest Tangrowth: 338-402 (83.6 - 99.5%)
252+ SpA Mega Pidgeot Hurricane vs. 252 HP / 252 SpD Assault Vest Tangrowth: 320-380 (79.2 - 94%)
252+ SpA Mega Pidgeot Hurricane vs. 252 HP / 252+ SpD Amoonguss: 338-398 (78.2 - 92.1%)
252+ Atk Heracross Megahorn vs. 252 HP / 0 Def Tangrowth: 344-408 (85.1 - 100.9%) -- 6.3% chance to OHKO
(cb megahorn kills)
252 Atk Tough Claws Mega Aerodactyl Aerial Ace vs. 252 HP / 252+ Def Amoonguss: 242-288 (56 - 66.6%)
252 Atk Tough Claws Mega Aerodactyl Aerial Ace vs. 252 HP / 4 Def Tangrowth: 222-264 (54.9 - 65.3%)
obviously it shouldn't be your switchin, but it can take hits from many mons 1v1 and do something back/pivot out
against suicune it can apply pressure with power whip/giga drain so that either crocune is forced to rest, or the sub set doesn't get a free sub.
on the amoonguss vs tangrowth, with one set (av) tangrowth has more or less the same bulk as a fully invested amoonguss in either def or spdef. granted, if it uses av tangrowth gives up sleep powder, but regardless bulk wise amoonguss's only niche over tangrowth is its neutrality to bug types, resistance to fighting, and i guess fairy?
you can also run like band/specs tan growth but thats way more niche.
basically tangrowth is good at what it does (tank), and should at least remain B-
also excuse my writing its like 3am