I don't see it as much scarier than tab-completion while programming or on a phone. As long as you're watching over it and fixing mistakes, it's just a convenience.
Tab compiletion while programming is hugely different then this completion and phone completion is somewhere in the middle.
While I am programming I usually know what method I want, I might not know the exact name but I know what it does and when I see it I select it. There may be a rare case where I want to see what methods are available but that is just a convient way to read "documentation". However this is guiding your sentence as you write it. I suspect that most people would have different phrasing if using this feature then they otherwise would.
If it wrote the entire email for you, and you watched it to see there were no mistakes, I suppose that would just be a convenience too. But at some point you cross a line and your voice is lost. That line draws closer, inch by inch.
I don't see it as much scarier than tab-completion while programming or on a phone. As long as you're watching over it and fixing mistakes, it's just a convenience.