Thundurus-T is one of UU's earliest bans; a good offensive typing, a unique Speed tier, very high Special Attack, and access to various setup moves like Nasty Plot and Agility, as well as great coverage in Focus Blast, Grass Knot, and Hidden Power Ice, was undoubtedly too much for UU to handle.
However, recent metagame trends have been working against Thundurus-T; the rise of fast Choice Scarf users like Krookodile and Hydreigon means offensive teams will usually have Thundurus-T in check. Slower builds also have certain tools to help combat Thundurus-T, as it struggles to beat Blissey without a Z-move, and other walls like Amoonguss and Sylveon can take even a boosted hit from full and retaliate back. Last but not least, Thundurus-T faces competition from other Electric-types like Zeraora and Mega Manectric, as they both boast a much higher speed tier.
Nevertheless, Thundurus-T would undoubtedly be really good in the metagame. Would it be broken, though? We are here to find that out!