Personally I dunno how well a mobile app can be used for updating firmware in a way that makes any sense. For instance, how do you choose the firmware files? Are they on your phone/tablet? You select them for upload thru the app? Good luck with supporting that.
I always pair a secondary controller with SmartThings to do firmware updates. I’ve used both HomeSeer Z-Flash (meh) and PC Controller (much better) to do the update. It works great, even with more complex devices like Inovelli red/black dimmers that have 2 different CPUs to update separately. My only complaint is when a device is paired with security, the update can take 5-10x longer due to the overhead, but thats not ST’s fault.