I own several smartwatches.. (Six) but have yet to find any smartwatch to reliably work by 'Voice' in real world environments, Noise and such...
What is more fitting for any smartwatch user is the ease to enter or display information on the wrist, not to replace your mobile device entirely at this juncture by the limitations in technology. The Samsung Gear S2 is certainly on the top of the list for attaining this use in technology.
If? and I do say 'IF'.. you are looking for any smartwatch to be totally reliant on voice specifically, then you won't be satisfied with any currently by their obvious technology limitations, but if you are looking for options in when a noise free environment, then the Samsung Gear S2 is not bad to only second to the Google Android Wear devices, I suspect this will change once the constraints in Google limiting the use of Google Now technology?
I suspect the main reason for Samsung to go with S-Voice is in payment by licencing fees for a product which is still new and not used in high numbers to justify lessening in the profits...
My Samsung Galaxy Note 4 S-Voice works adequately, but not as good as Google Now or better than Microsofts Cortana environment in which in my opinion is better than Googles Now technology.
In reality, I use Microsoft 'Cortana' on my Samsung Gear S2 in which opens on my smartphone (Galaxy Note 4) thru my Bluetooth headset.
I use Tasker for relating to any voice commands I may give, but that is a more complicated manner for most users.
I gave harsh criticism of the Samsung Gear S-Voice version in duly earned for it! as it has problems, but the device itself has many other beneficial uses over the others.
Smartwatches are a very personal device merited by personalities in what people will like or hate in each. What I like may not be things you like or vice versa.
Just to show in that S-Voice does work on the Samsung Galaxy Note 4, I dictated this, ( not easy?, but doable).
The hardest part is to remember all the shortcuts or tips & tricks,
There is much on the Internet in especially Youtube on how to make best use of S-Voice and even here
Top 5 tips and tricks for using Samsung S Voice | Android Central
Like everything in technology in there will always be some learning curve to it.. and varies person to person.