That is seriously great to hear! Before SmartThings had the resources to ramp-up the Support team, we’d “too often” get your initial impression. Support has always been friendly and excellent at handling the issues under their control, but when they, legitimately, suggested the Community for assistance, they unfortunately, often / sometimes / usually (?), did not followup to ensure the Customer issue was eventually resolved, or at least assured that it was noted as “unsolved”. Hearing that they have closed this service-gap is very encouraging and a sign that SmartThings is growing resources in the best possible way. (@April … Kudo’s to “Blake” in Support … and/or praises to the whole team for squeezing out that extra bit of customer service quality!).
It is important for SmartThings to distinguish unsupported Devices, since there are, nearly literally, an “infinite” number of Devices out there. Yet, as you noted, there is always a possibility that the unsupported device isn’t the actual root of the problem. SmartThings is a complex platform.
I hope the Community can chip in and help, though, but, as @JDRoberts has mentioned, we have no way of knowing the details of your configuration (device types & SmartApps (official, and custom), various Event and Activity Logs, debug traces) unless you share it in detail. Of course, this is where Support has an advantage, since they can access your entire profile.