Since smartthings never gives you an entirely local operation (for example the app and notifications always require the cloud), it’s more about assessing each individual use case. There are two different use case categories to be concerned with.
One) everyday operations when the cloud is available as expected. For this situation, you probably don’t really have to worry about local versus cloud as long as you are satisfied with the QOS (quality of service) of your existing set up. If you do have some sensors or some switches that seem to take a long time between physical event and expected outcome, you might want to see if some of those can be moved to more local operations.
Two) planning for outages. There will inevitably be sometimes when the smartthings cloud is not available or the Internet is out. What, if anything, do you expect to still work from your smartthings set up then? We have an FAQ that discusses this in detail, but topline I think the most important issues are three-way light switches, light switches that control smart bulbs, Motion sensors that trigger lights, and security sensors that trigger a local siren. These are all things that you may specifically want to have continue to operate even if the Internet is out. Use cases where a physical event in the environment, like someone walking into a room or physically flipping a switch causes something else physical to happen in the environment, most typically a light coming on, but may be a siren.
Notice that none of these are use cases where the trigger is voice control or an app or even a schedule.
For these kind of physical event triggers, I think it generally is useful to move them to local operations so that they will continue to operate as expected even if the Internet went out.
Again, see the FAQ: