Got this response from tech support:
Thank you for reaching out to SmartThings again in regards to your Leviton Decora WiFi switchces and dimmers frequently disconnecting.
We are escalating this ticket in order to further investigate. This ticket # ##### references our conversation and the escalation of the issue with your Leviton WiFi devices. Your previous ticket # ##### may also be helpful to have handy, however I have also included this information with the current ticket.
I am terribly sorry for the understandable frustration that you are currently experiencing with the Hub and with the new SmartThings app. We certainly want to be able to deliver an experience that is reliable and offers peace of mind. Clearly, with your current situation, that is not the case. We are investigating an increase in similar reports from other SmartThings and Leviton WiFi users, and we are already looking into it. We will reach out once we have more information.
Your patience and assistance are tremendously appreciated. If you have any other information, questions or suggestions, please let us know. We are here to help.
Technical Support Agent