Yes lasting repairs of a critical failure of a frame of any material is generally cost-prohibitive. Many small repairs (say stops and bosses) can be done quickly and effectively on all materials but nonetheless requires specialized skills and tools. There is a wide spectrum of possible failures and repairability should be down in the list of priorities. If you have a support crew it makes sense to carry a second bike; McGyver repairs done on the fly will not last.

For an endurance ride you need a bike you are familiar with and that fits well.