The stress on plastic ferrules on brake cables is really not all that; if the ferrule is fully seated on the cable, it's all supported by the housing underneath it. In the past seven years, I've built up seven bikes for myself and family, from bare frames. Five of them have had plastic-ferruled brake cables, and I've never had a problem.

If the ferrules are not sliding into cable stops smoothly, don't force them; spread the stops slightly with a small screwdriver until they DO slip in smoothly. (Remember: slow and careful, ALWAYS, with bike assembly)