well, technically speaking they run out fuel really quickly and gold farms exist, so gold really not an issue if you can set one up. Plus Witch Farms would give you infinite redstone, plus whatever else a witch drops, leaving only Iron which is fairly easy to get a hand on.
A lot of people prefer powered rails over furnace minecarts because of that. Plus furnace minecarts a just slow and need to be constantly refueled for long distances. You can only refuel them with coal and if I remember correctly only by hand as well.