What are the specific brands and models of the devices that don’t work? And are you using the UK version of the smartthings hub or the US version?
The basic compatibility rule for zwave
SmartThings is a certified Z wave controller, but that only means that you should be able to pair any certified zwave device to it that uses the same zwave frequency as the hub and smartthings should be able to send what is called a “basic” command to it, which is essentially an on/off.
going beyond basic
For any features above that, there needs to be a “device type handler” (DTH) which can send the specific commands that that device expects.
There will be one for the devices on the official compatibility list. And in many cases if the device is similar to one on the official list, you can just tell smart things to use that DTH and everything will work fine.
If the device has unique features, many community members have written DTHs that may work, or can help you write one that will work.
So I’m really surprised that support would have said that.
The only time you can be certain that a Z wave device would not work with your smart things hub is if that device is on a different frequency. Zwave uses different frequencies for different regions so, for example, there are devices you can buy in the UK that would not work with a US hub and vice a versa.
Also, it’s not always possible to put two controllers on the same network. They could confuse each other.
But other than those two issues (frequency and multiple controllers) most certified Z wave devices can be made to work with SmartThings.
So again, if you can let us know the brand and model, we can probably figure out what the issue was.