OK, fair enough.
1) As has already been said, they can misfire. I think the odds of this happening are remote, but it remains a possibility. It could also fire during an aircraft emergency (again).
2) I'm a firm believer in KISS. I only like to add complexity when necessary. It could cause a one in a million unforeseen conflict w/another component.
3) They're hard metal boxes firmly strapped right across one's spine. Poo Happens. If there's a canopy collapse while landing, or a hard free fall collision. That small metal box could snap your spine, transect your spinal cord, & leave you paralyzed.
I've already been in a situation where I couldn't re-orient my body prior to impacting the ground. It's not a good feeling. Had I landed on my back, I probably would've been left paralyzed. It was a student jump, w/an AAD.
4) A Cypres2 would run me about $1800, including scheduled maintenance. While easily do-able for me, it's still a fair piece of change.
5) When I look at the Save Lists of the two major players, I'm not sold. Vigil's list has a number of repeat incidents listed. They're double-counting some of their saves. I haven't noticed that on Airtec's list, but it's beside the point. What I have noticed is almost all the saves are of students. Student jumpers making student mistakes. Even a fair number of the experienced jumpers' saves are of the same cloth. For example, one guy was uncurrent from a winter, & sucked it all the way down to activation altitude without realizing the ground was getting kinda close . They also include multiple jumpers who did pull silver, but their units still fired. Those people did save themselves.
6) Surprised to say, but there is also the enjoyment of "Jumping Naked". I've got absolutely nothing against anyone who jumps w/an AAD. Comments about their not being real skydivers were asinine, & not made by me. When I rode motorcycles, I always wore a full-faced helmet, & full body protection. I would've been a nervous wreck not to. But, in the air I feel a purity of knowing I have to pull or die. I'm not a reckless person, but do enjoy this aspect of the sport. Perhaps swoopers feel a different version of this? Dunno.
We average a little over three million jumps per year in the US. I'm guessing Europe does the same amount, or even a bit more. Out of six million-ish jumps per year (not counting the rest of the world), the lists show maybe 3-5 legitimate LOC saves. Almost all of those are from higher risk jumps like tracking dives. Last year, I think only one person got taken out from behind by someone swooping into them in free fall (non-tracking dive). The report was unclear as to the circumstances. That's one jump out of six million (plus, Asia & down under) jumps. I think those odds are so remote that it's more likely for me to have one of the other problems I listed above.
When an incident like the one that prompted this thread occurs, it gets everyone's attention. That gentleman had more skill & experience than I'll probably ever have. Everyone who knew him has no idea how this could've happened to him. A health emergency? Bird strike? We'll probably never know. Clearly, it was something extraordinary.
How often does someone of his caliber go in, though? I can't remember a single instance that wasn't explainable by some factor like a mid-air collision.
A lot of injuries in this sport go unreported, but not the deaths. The odds produced by those save lists don't make a compelling case for me to strap an AAD onto my back. I limit the types of jumps I'll do, & keep aware of whom I'm jumping with. I think I'm better off without one. I was taught years ago that everyone has their moment, no matter who they are. That's very true, but I still can't see myself not pulling unless I'm unconscious or seriously incapacitated.
I do think the proof is overwhelming in support for AADs being mandatory for student jumpers, though.
P.S.: After having listed all of this out, if I am that one in six-ish million who gets knocked out & goes in, next year? I'm gonna come back, & kick the piss out of you, Krisanne .