Galahad has a microchip between his shoulder blades that the shelter implanted with all data proving that he is mine and if he is lost needs to be returned to me (and I think maybe his medical record, too?). Even if his old owners (whoever either turned him loose or lost him a over year and a half ago) come looking, that little chip says that he is MINE. It's kinda sad to me that one little microchip can override years of ownership. (not that they bothered to look for him, obviously, since he was in the shelter for months)
They're starting to discover that those chip implants in longer-lived animals (horses and livestock in particular) are starting to cause growths (some malignant) in the implant site. They don't know if it's specifically that brand of chip that does it, or if it's just that sort of thing being in their body for 20-30 years.
Now. People live a lot longer than horses do... :/ This isn't even touching into the hacking and virus parts. This is just the biological aspect. How are they going to keep EVERYONE'S body from rejecting the implants, especially if they're not biological material? Surface piercings will ALWAYS eventually reject and start to migrate, dermal implants don't always come out looking so pretty... There's also just the issue of them breaking. I read an article on stumble once about someone who implanted a small magnet in his finger to see if it would give him any sort of extra sense (which it did, to a degree interestingly enough), and then the magnet SHATTERED in his fingertip. Potential for broken bits to end up in places in your body they just should not end up. Like your heart tissue for instance! He got lucky in that since it was a magnet, it kept itself in the same general location, but that's still a lot of pain.
What hackers can do now is already scary. Store Support, wherever the hell they are, can hack EVERY SINGLE MACHINE at my work from anywhere. All our scales, our printers, our computers, our accounts, probably the cameras; everything but the registers, which are owned by a private company. They can also accidentally wipe all record of you ever working there... >_>;; I'd be a lot more worried about it if I didn't need them to hack everything remotely all the time because I can't even figure out how to get the label printers to spit out a test strip. XD However, what the hell's stopping them from when they get asked to fix a store security camera, spotting an employee or customer they think is attractive and watching them all day? *shudder*