Wouldn't it be unethical to take on a job you were certain you'd fail at? (If you disclosed that, and they hired you anyway, then there's no ethical problem.)
"Fairly certain", though. If her odds of saving Yahoo were twice as good as an alternative CEO, that was a possibility worth a small fortune. And presumably, they knew the odds were low after sacking the last few CEOs. I'm hoping that she didn't even have to say it - a competent board could approach the whole thing with "we're in deep water, we think you're less doomed than most people would be".
That may well have been the case, but if those odds are 1% and 2% I wouldn't want my compensation to depend on me saving the probably-doomed company.
It's perfectly ethical if you've had a frank discussion with your employer about your odds of success. If you sold them the idea that you could save it, but privately were certain you could not, that is plainly and simply unethical.
