At this point, itâs likely that 90% or more of the people using the smartthings app donât have a hub at all. Itâs 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 donât 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 isnât about using it as a voice assistant: there are devices that can work with Alexa âroutinesâ 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 thatâs what you prefer.
So while the hub does open up the possibility of using additional devices, itâs 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 youâre waiting for smartthings to figure out what they are doing with regard to hub hardware in the US. 
At my own house weâve been in a âhub optionalâ configuration for over a year because the ST system just wasnât reliable enough for what we needed. We do all of our âmission criticalâ use cases with appleâs 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, itâs probably best to start looking at alternative hubs right now if you donât want to wait.
But if youâre open to other possibilities, there are a lot of devices that work now with smartthings, no hub required.