Mega Charizard Y is excellent. Drought weakening Surf for allies, Earthquake immunity, access to Tailwind, Will-O-Wisp, however Charizard Y's usage stems mostly from its amazing offenses. Heat Wave boosted by automatic sun and STAB coming off of 417 Special Attack destroys the common Amoonguss, because it is super-effective and makes Rage Powder completely useless due to the fact that it hits BOTH opponents. Again, Tailwind and Will-O-Wisp are nice, but that role was perfected by Talonflame in gen 6 and Incineroar and Whimsicott in gen 7. It also has a very good matchup against most of the Pokemon in doubles, like Metagross and its mega, Whimsicott, aforementioned Amoonguss, Hitmontop, Aegislash, among others. However, its 4x weakness to the super-common Rock Slide along with a speed tier that is only average definitely hold it back. It is placed in Tier 2 on the gen 7 Doubles OU viability rankings for that reason, just short of top tier.
Smogon's Gen 7 doubles analysis of Mega Charizard Y:
A high Special Attack combined with the weather-setting ability Drought makes Mega Charizard Y one of the strongest attackers in the tier. Mega Charizard Y also boasts a strong matchup against Metagross, one of the most common Mega Evolutions. With its base 100 Speed, Charizard can outspeed Genesect, Tapu Lele, and Kyurem-B but still requires speed control to perform most effectively. Offensively, Mega Charizard Y struggles against opposing Fire-, Dragon-, and Water- types, as they can usually switch into Fire-type attacks relatively safely. Defensively, Charizard struggles against common Rock- and Electric-type attacks and is weak to physical attackers due to its less-than-stellar Defense. However, despite its flaws defensively and its middling speed, with sufficient support, Charizard can perform its offensive role excellently.
Charizard's VGC 2018 analysis:
Mega Charizard Y is a dangerous spread attacker, dealing massive damage with Sun-boosted Heat Waves. Unlike most Fire-types, Water-types have trouble with Mega Charizard Y because of Drought and Solar Beam. However, Mega Charizard Y heavily relies on the sun to deal damage, and it has an unfortunate weakness to Electric-type attacks, and a 4x weakness to the ever common Rock Slide.
Best checks and counters:
Tapu Koko, anything faster with Rock Slide (ex. Garchomp and Scarf Landorus-T), rain setters like Politoad and Pelipper and rain abusers like Ludicolo and Kingdra, anything with enough bulk to take a hit and retalitate with Super effective damage.
Hope this helps!
