Yes there's a lot more that can go wrong with rebreathers. Open circuit is much more forgiving. As long as you can breathe you can stay alive. No single mistake will kill you and there are multiple opportunities to fix any problem. Whereas with a closed circuit rebreather there are failure modes which are essentially "silent" (or at least easy to overlook in a stressful situation) that can kill you quickly. Most rebreather deaths involve some sort of diver error, but I don't trust myself to be perfect every time.