Right now the biggest challenge to home automation is well, automation. In my opinion, nobody, and I mean nobody, has figured out how to do automation. Whether you use Smartthings, Hubitat, Wink, HomeAssistant, etc. you pretty much have to think like a programmer to build any kind of real automation. That kind of thinking is way above the level of the average Joe-customer. I think that Iris, for all of its faults and limitations, probably comes the closest. It has the easiest out of box experience, a very limited, predefined set of automation rules written in natural language make it easy for newcomers to understand. But even so, it's not really automation it's home control.
What doesn't exist yet, but will in the very near future is a true "smart home" hub, whether it is a stand alone device, or part of another. Here an example of what I'm thinking..
A customer buys a motion sensor, opens the box, holds a QR up to a camera on the hub (or via bluetooth NFC). At that point the system connects the device and asks the user what room he's putting the device in. The customer installs the device and confirms (by voice) the device is installed. The smart home system responds, informing the customer there's already a light switch in the room, and asks if he would like the lights turned on when anyone walks in the room. With a simple yes or no response, the customer has already began automating the room.
How does this happen today? The customer logs into a mobile app or web site, clicks a button to begin pairing a device, opens the box, pulls a plastic tab then waits for a few minutes, hoping the device actually is recognized. Then the user installs the device and returns to the mobile app/website, looks for the appropriate "rule engine", not understanding what they are, or which one is most appropriate for the task. The user then follows a very complex set of nested menus and options, scrolls though list of devices to find the newly connected device, and finally, after several minutes of frustration, completes the task.
For many of us, especially power users, this kind of detailed programming is second nature and not all that difficult. But for most consumers, it's a non-starter.
What the "mass-market" hub of the future looks like? That's anybody guess. But I bet the letters A and G will be huge players in that future.