The phrase 'supposed failure' is correct and usable in written English.
You can use it when you are describing something that was anticipated to fail but did not. For example, "His supposed failure to receive funding turned out to be a success as he was able to get the money he needed."
Goetz convincingly refutes myths about the supposed failure of public housing.
Instead, the suit hinges on his supposed failure to "diligently supervise" the firm as it raided the client accounts.
Gayle is angry with the supposed failure of the Caribbean's current cricket administrators to support their players.
And the fact that so many people were quick to celebrate her supposed failure simply complimented the album's unflinching temperament.
So naturally Romney tried to go after Obama not for what he did or didn't do, but for his supposed failure to talk tough enough.
But elsewhere he repeats popular theories (about the alleged corruption of politicians, or the supposed failure of Mexico's free-trade deal with America) without really probing them.
In addition, plenty of newspaper leading articles since the EU referendum have scorned liberalism for its supposed failures.
Or will it simply turn into one of venture capital's supposed failures, becoming a fairly profitable site that just ticks over?
Unlike many supposed failures and fears associated with increased global integration, in the case of the environment, globalization has had a part to play.
Clark's uncle equates Bill Clinton's roaming eye with Hillary's supposed failures as a human being; his takeaway is not to trust any woman who wears a pantsuit – never mind that Clark wears pantsuits to her job.
He rose to the podium in the middle of a raucous debate, filled with blistering attacks on his supposed failures as a statesman, and informed Parliament that he was "not blind".
