I crossed the GofM, and the Caribbean Sea many times without a true backup (just a bolt on wheel drum to do sheet to wheel if need be) but my autopilot was fairly new and I didn't push it hard. I also hand steered a lot.I always thought having a spare self steering system was pretty standard for anyone solo sailing on the ocean.
Once I decided to head west around the world though I bought a whole new autopilot system that is exactly the same as the one installed, so I could just replace a failed part in minutes and be back at it in no time. I feel much safer now, especially after balancing my rudder so the autopilot doesn't work very hard at all, even when I am pushing he boat hard.
In my opinion, it is the rudder that is most important but mostly overlooked. All boats these days are built with unbalanced rudders as sailors, especially racers, like the "feel" of it and builders also like the safety thing of it swinging back to center when unmanned. I think a boat doing any serious long offshore sailing needs a balanced rudder to take the pressure off the steering gear and helmsman. It is the pressure from the unbalanced rudders is what is causing so many problems with autopilots.