Some of the more advanced features of QMK aren't available in Kinesis' onboard programming, although it has become increasingly capable and I'd expect this version to be even more capable.
I'd suggest if you're interested to check out the QMK Docs [1] and specifically the "Software Features" where many of the features QMK has that SmartSet and Kinesis' onboard programming don't support.
Wow. Lots of interesting things. But how much of this do you actually use? I personally have a hard time imagining using any of it as part of daily workflow.
Personally I use very little of much of these big features. One that is indispensable to me is Mod-Tap, which allows me to use normal keys as modifier keys. For example, my 'z' key also functions as "Ctrl + Shift" when held down. My Caps lock key functions as "Ctrl + Shift + Cmd + Option" when held down and an Escape key when I tap it.
This actually is a feature now on SmartSet (I believe), but it wasn't always. Kinesis is pretty good about slowly iterating new features out to people in its custom firmware.
Combos is a feature that I'd like to explore more but it doesn't cooperate with mod-tap keys so it'll require some more thought. I've used mouse keys in the past regularly though lately haven't been using it very much.
The broader point is that it at the minimum offers me things that would require me to use something like Karabiner Elements or AutoHotkey to accomplish less well in software on a firmware level. I can change just about anything I want on my keyboard to make it my own without having to be concerned about configuring it on the operating system. It's also just fun!
I'd suggest if you're interested to check out the QMK Docs [1] and specifically the "Software Features" where many of the features QMK has that SmartSet and Kinesis' onboard programming don't support.
[1] https://docs.qmk.fm/