The Samsung Wi-Fi appliances are handled via the smartthings cloud, not the hub. Same ST app, so they can be combined in automations, although I do see some disappointment about the level of integration, I just don’t know the details.
As far as which app to use primarily, smartthings allows for much more complex rules than HomeKit does at present, but is less reliable.
I personally use HomeKit for all of my critical use cases, including things like turning lights on at sunset. I use smartthings for complex convenience notifications, like receiving a notification if the guestroom window is open, rain is expected, and the guest is away from the house. But I myself put reliability at the top of my requirements list now, because I am quadriparetic, and home automation is not just a fun toy for me. Also I may not have as many “Plan B” options as someone else. So there are a lot of individual factors that go into deciding which rules creator you want to use.
See the FAQ:
How to Get Started Creating Complex Rules in SmartThings (2017) (SmartThings Classic)