Trying to turn around a failing technology company is almost always a futile task — just ask Marissa Mayer at Yahoo or whoever it is who now runs BlackBerry. In the months that Zenefits has been rebuilding itself, a spate of competitors have set their sights on its business. One of them, a well-financed start-up called Gusto, has put ethics and culture at the core of its appeal, and it has been scooping up disaffected Zenefits customers.