>Their other products seem to be well-regarded, do they not?
Not entirely, no. There's a history of Zenefits customers and their employees suffering from serious technical and support failures.
http://www.buzzfeed.com/williamalden/zenefits-hr-rocket-ship...
>in late February this year, with coverage supposed to start days later, Zenefits informed Hawkins that it had made a significant mistake, attempting to enroll his employees with an insurance provider that didn’t cover the company’s region. The insurance wouldn’t come through as planned. [...] In several cases employees like Harris, who had put their trust in Zenefits, were left without health insurance for a month or more after they had expected it.
My 15-person company uses zenefits. When our HR person was sold on zenefits she was promised some features that, bluntly, didn't and don't exist. They also, as a user, required some emailing to get stuff set up. It seems to be ok now that it's all set up, but it definitely wasn't 0 friction initially.
So, yes, it's hard to build. That doesn't mean that no one can do it. Many companies have done so, including several startups. And, given how poorly one of the market leaders (Intuit) has implemented it, I'm willing to give a startup a try.