I use a Verizon 551L USB 4G LTE modem stick with a Peplink router as my backup ISP. SmartThings seems to operate fine on this connection. Lights triggered by motion sensors trigger a little slower but I believe there is full functionality.
Verizon blocks incoming traffic in certain ways. Maybe specific ports + some other rules. I know you can’t use their 4G modem/routers on devices like remote camera/DVR systems where you would want to log in remotely to a web server and view or copy video images. And I don’t believe you can bypass this by changing web serving ports etc. I could be wrong but I believe they are trying to block specific incoming uses to protect the user and mobile devices. SmartThings and Thermostats establish their own connections and not impacted in most cases. Maybe you would get some degraded connectivity with cloud to cloud or rest endpoints stuff if the implementation required making requests to the hub itself. I haven’t thought enough about how those smartapps and devices work to know for sure.