There are still several rebreather failure modes due to user error or equipment failure which can kill the user and won't necessarily be detectable by a self test. So although rebreathers can be made somewhat safer they will never be as safe as open circuit for sport diving. At least with OC gear, if anything goes wrong the problem is hard to miss and the solution is always simple.
It's tough to switch if you're already unconscious due to hypoxia. You really can't notice the onset of hypoxia fast enough to fix the problem. In theory that shouldn't happen if the diver sets up the gear correctly and pays attention to the alarms, but in practice sometimes divers make mistakes and end up dead.
There was an incident in my area several years ago where a rebreather diver missed a step in his pre-dive checks and jumped off the boat with his oxygen valve off. By the time he swam to the anchor line he had consumed most of the oxygen in the loop (while probably not noticing the alarms), passed out, and drowned. That was user error but the fact remains that if he had been using open circuit gear, a simple error like jumping off the boat with a closed tank valve almost certainly wouldn't have killed him.
No you will not sink anything like a stone as long as you have a properly balanced rig set for neutral buoyancy with empty tanks. Negative buoyancy at the start of an ocean dive is around 11kg at most. The problem will be obvious and unless you totally panic you can swim back to the boat, or reach around and turn on a valve. You won't just suddenly pass out, like what can happen if you make a series of bad user errors with a rebreather.
