At this point, it’s likely that 90% or more of the people using the smartthings app don’t have a hub at all. It’s only necessary if you want to use zigbee or Z wave devices. (And not 100% necessary for those, but that requires a different server and a lot of technical expertise.)
As @ogiewon mentioned, there are lots of devices that you can use with the smartthings app that don’t require the smartthings hub, and many are excellent. In addition to Lutron Caseta Switches and bulbs that work with the Philips Hue bridge, there are LIFX bulbs, the August Wi-Fi lock, Meross smart plugs and garage door controller, Switchbot button pushers, and many more.
The only device category which is really missing is small battery operated contact sensors and even there, you have some choices, particularly if you use Amazon Alexa as an intermediary. (This isn’t about using it as a voice assistant: there are devices that can work with Alexa “routines” without having to speak to Alexa.)
Some of these will require having their own bridge device to connect to Wi-Fi, but once they are set up you can do daily management and create rules through the smartthings app if that’s what you prefer.
So while the hub does open up the possibility of using additional devices, it’s certainly not necessary to use smartthings, and there are alternatives for pretty much every device class, sometimes very good alternatives, if you want to go ahead and get started while you’re waiting for smartthings to figure out what they are doing with regard to hub hardware in the US.
At my own house we’ve been in a “hub optional“ configuration for over a year because the ST system just wasn’t reliable enough for what we needed. We do all of our “mission critical“ use cases with apple’s Homekit now. But we still use smartthings for some WiFi devices, including Meross smart plugs and Switchbot button pushers.
So it just depends on what direction you want to go. If you want to jump in with Zwave, then, yes, it’s probably best to start looking at alternative hubs right now if you don’t want to wait.
But if you’re open to other possibilities, there are a lot of devices that work now with smartthings, no hub required.