I would think that simple devices are easier to secure. They don't have the same horsepower available for encryption/decryption, but they have far fewer vulnerable spots than a complex system. I doubt that anyone knows how many points and methods of potential entry there are for a typical PC.
A blue tooth device may have only one point of entry and only one protocol to defend. If the 8-bit MCUs don't have enough power to be secure, even at that level, maybe the low-cost 32 bits will be able to make greater inroads by meeting that requirement.
Replay available now: A handful of emerging network technologies are competing to be the preferred wide-area connection for the Internet of Things. All claim lower costs and power use than cellular but none have wide deployment yet. Listen in as proponents of leading contenders make their case to be the metro or national IoT network of the future. Rick Merritt, EE Times Silicon Valley Bureau Chief, moderators this discussion. Join in and ask his guests questions.