I forgot to mention one more thing… I myself don’t have visual challenges, but I do use text to speech and other utilities designed for people who are blind because I don’t have much use of my hands. In particular, I rely on voiceover on my iOS devices.
In April 2017 (over a year ago) an update to the SmartThings mobile app broke voiceover compatibility. The company was aware of this and said several times that they intended to improve it, But it still doesn’t work well. So your family members will likely not be able to use the SmartThings mobile app at all. I don’t know whether that matters in your household or not, or if you intend to have them do everything by voice.
In contrast, HomeKit works very well with voiceover.
Also, pretty much everything, but in particular any custom voice features that you add to SmartThings, will require that both the Internet and the smartthings cloud be working. HomeKit works locally except for voice processing, and you can do the voice processing via cellular as a backup option.