Whats to prevent someone from connecting this to their smart door locks? Then if that person has a big party, and someone with bad intentions shows up, they can simply say "Alexa, at 3:30am tomorrow, turn off the alarm system, and unlock the doors". Or "Alexa, turn off the dialysis machine".
Sure Amazon might make it more secure in the future, but probably only after enough of these unintended consequences stack up.
I also understand that the average HN reader is probably not rushing out to introduce a single point of failure into their home security system. My point is that Alexa is essentially saying "don't worry about how it happens, just dream big about what awesome stuff might happen".