The phrase "unassailable advantage" is correct and usable in written English.
You can use this phrase when you want to describe something as being difficult to challenge or impossible to overcome. For example, "He was playing chess against a grandmaster, so it was an unassailable advantage for his opponent."
After the break Inglis, Scott and Dugan quickly gave Australia an unassailable advantage.
Until recently, Europe's biggest derivatives exchange spurned links with its rivals, confident that the liquidity of its trading floor gave it an unassailable advantage.
At 21 he had reason for his flamenco snap, after winning three out of four points with Ballesteros as Europe built an unassailable advantage going into the singles.
The assumption about playing at the Camp Nou is that the vast playing surface presents the home side with an almost unassailable advantage.
Both involve a creativity so unexpected, so sublime or shocking, that it gives the practitioner an unassailable advantage over the opposition.
Rather than competing in the market for holidays lasting a fortnight, where the Mediterranean sun has an unassailable advantage, they now fill their beds by depending largely on people coming for weekend breaks.
"It all adds up to an unassailable competitive advantage for Google".
New polls showed him losing his edge on the economy, once thought to be his unassailable competitive advantage in the race against President Obama, and lagging behind in crucial swing states.
After Desert Storm there was talk of a "revolution in military affairs," a new order in which precision guidance would be combined with advanced sensors and increasingly sophisticated information and communication technologies to produce an unassailable battlefield advantage.
Because of its domination of the global search market and ability to penalize competitors while placing its own services at the top of its search results, Google has a virtually unassailable competitive advantage.
(A pessimism, I should say, inherited from my father, who has always tended – mostly justifiably – to assume the worst even in the face of unassailable sporting advantage, and which he in turn had adopted from his own old man, who had understandably first lost his faith in myths of native superiority aged 15 at Gallipoli).
