Same with any of these assisted braking devices. "Anti-Panic" is related to a natural response to grab things when startled - the GriGri+ will disengage and stop you then also.When I let go of the handle on the Beal Birdie while rappelling it comes to a stop. Same thing would happen if I got knocked out.
While braking without user input may be a positive (unless it induces suspension trauma worse than whatever fall is prevented in some particular case) "what will happen after I'm knocked out" is NOT a really primary risk that we face. The primary risk is all the stuff that could lead to a high-impact event in the first place.
And the Safeguard and GriGri+ in toprope mode are designed to engage with little to no tension on the rope (vs lifeguard and grigri which will tend to feed some slack before engaging - I don't know the behavior of the birdie in that regard.)