I recently upgraded to Galaxy S7 with S Voice installed and I have encountered 2 major flaws with S Voice.
1: S Voice doesn't see any of my see my phone contacts. The initial contacts were imported from my gmail account I thought that might be the problem, So I did a phone reset and did a transfer of contacts from my Galaxy S5 and still get the same response " Can not find that name in the Contacts"
2: the "S Voice" voice assist interface is not very secure my wife was able to engage "S Voice" after a few attempts of repeating my secret code,
On the other hand my wife has a iphone 6 plus which uses siri voice assist, the default access is "Hey Siri!" only her voice can activate Siri, I tried several times with no success, their voice recognition software is far superior to "S Voice" Samsung Step up your Game!!!