It can be used as a launch pad into the rest of your network as IoT devices are notoriously insecure. There was a case a few years ago where a bank was hacked via the internet connected thermometer in the fish tank.

One option (which is what I do) is to create a separate WiFi network for your IoT devices that is isolated from the rest of your network. A lot of the modern routers have the option of a 'guest' wifi so you can enable that and use it for the IoT devices.

You should also check for and apply any security updates/firmware updates for the device and then regularly check for the same.