I use Gusto and like it for it's simplicity. It makes a complicated part of running a company much smoother. That said, I'm very curious what exactly all those engineers are doing every day. What's the most meaty part? Payroll? Dealing with govt? Keeping the servers up? I think unexpected scaling pains are one of the most interesting and unsolved issues with being a CTO and would love to hear more about that at Gusto…
I used to be a PM at Gusto so I can add to Eddie's answer. As Eddie mentioned, it's non-trivial to keep the existing applications running as well as they do. That being said, a non-trivial amount of time/resources is spent making an already amazing product that much better. For example, my friends just launched a new product that lets you pick your payday (https://techcrunch.com/2018/06/21/gusto-flexible-pay/). To get this to work, there's a lot of data science involved to make sure you're not exposing Gusto to financial risk (it's effectively lending money). Since taxes are withheld with every paycheck, the payment flows associated with flexible pay become incredibly challenging. They're an amazing team. Go Eddie!