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.