The problem there is that VoIP traffic is much more sensitive to network bumps and potholes than most general office traffic. That translates to garbled conversations, difficulty connecting over Wi-Fi, or (worst case) dropped and lost calls. If your business is small and your network is essentially contained in one or two wireless routers, then your configuration and testing headaches might be fairly easy (though still there). For medium and larger networks, these tasks can be complex and time-consuming, which translates into added cost in terms of man-hours.
Company Description: MightyCall’s virtual phone system is designed for small businesses. Their system makes it simple to set up and start using their virtual phone system. First, you pick your business phone number — either toll-free or local. Then you add your employees, set your call routing rules, and begin accepting calls from customers!

That's the basics of UCaaS, but the concept is constantly evolving to include more communication and collaboration technologies. Those capabilities also get tweaked to provide new benefits, sometimes general, sometimes aimed at specific verticals, like healthcare, for example. The key is integration. Voice is becoming integrated with other back-end apps, and UCaaS is making that easier. In fact, it's become so popular that it's seen rapid growth over the last several years as recent research from Statista bears out.
Unfortunately, some businesses fail to realize just how much each interaction matters.