'a major advantage' is a correct and usable phrase in written English. You can use it to describe a significant benefit. For example, "The use of cloud-based computing offers businesses a major advantage in terms of flexibility and scalability.".
Nevertheless, he saw Trump's buy-in as a major advantage.
The inclusion of a libretto alone is a major advantage.
Corbyn's proposal, unlike orthodox monetary financing, would not add to the national debt – a major advantage.
Democrats see Trump's inexperience on national security issues as a major advantage for them.
Our skills are complementary and we trust one another – a major advantage.
There, Snow has a major advantage: Snapchat is blocked in China.
This would be a major advantage amid the chaos of battle.
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Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia