For Z wave, you can use any certified zwave switch for basic on/off. If the switch has additional advanced features, you might need to get the community’s help to build a custom device handler for it. Make sure you check on the official Z wave alliance product site to make sure that the device actually is certified.
Zigbee is more complicated. Zigbee allows for many different profiles, and they cannot all work together. The standard also allows for the use of proprietary coding which means many manufacturers can make devices that will only work with their own controller.
Smartthings uses the standard “zigbee home automation” profile (ZHA 1.2 ), so if a device is certified for that, you should be able to use it with smart things. If it is certified for a different zigbee profile or not certified at all, you may be able to pair it to smart things but not get any functionality from it. Or it may pair and work for an hour or two and then drop off the network again. You will find a number of conversations in the forums from people who’ve tried various cheap Chinese devices that were not certified for ZHA and had these problems.
I have personally had a lot of difficulty with customer service departments from various companies telling me that yes, a device was certified for zigbee home automation when it wasn’t. That’s because they don’t know what certification means. So if the device is zigbee and it’s a home automation device they will often tell you that it is ZHA certified when you asked, even though it isn’t . It’s best to ask to see a copy of the certification to be sure.
The other issue, of course, is electrical safety. Different countries have different standards. I myself only use UL certified devices, but I have particular concerns about fire safety. For the UK or Canada you would be looking for a CE certification.