I've not seen any indication that Zenefits is offering a shoddy product. Their other products seem to be well-regarded, do they not? And, their other products are in the same general area (most benefits programs have some tax and accounting repercussions that they'd need to understand, as well as integration with banks and payroll systems), so they already have some domain expertise in-house.
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.
