Calcium reactors can fail, but even when they do, it is nearly impossible for them to crash a tank. The are not too easy on the pocketbook, though and are even $250 used. The doing pumps are the point of failure with kalk or 2 part, most of the time - timers and switches can/will fail and if they stick on, you are done... if they stick off, your tank is still fine. You can make a simple kalk doser out of a 5G water jug and a John Guest ball valve that, at worst would just clog and fail to dose. Manual dosing of 2 part is pretty reliable.
IMO Calcium Reactor is by far the safest, but they are not for everybody and are not a good fit on small tanks.