Busses breakdown too. I mean yes the monorails are breaking down more often, but its still not like they are down every day. I understand though about the fear of heights.
Just be prepared for it to take over an hour to get to Epcot every time you go. If Epcot isn't big on your list it shouldn't be a huge problem, but if you plan to visit a lot during your stay you might want to think about picking a different resort. The Poly is great, but if you have issues with monorail transportation, there are other really nice deluxes off the monorail system that you might want to consider where you won't be paying a premium for that monorail access. You can walk or boat to Epcot and DHS from BC/YC/BWI and you will have busses to take you to MK and AK. Alternatively you could stay at the Poly and hope for the monorail system to shut down while you are there so Disney has to replace it with busses.