If the end are not flat, more so for the gears, then you can have flex in the system. Ferrules can not stop that. If you use a cable cutter pliers the proper way is to file them flat (or maybe on a slight angle for gear if you know it's bending a lot, perhaps for the rear..) Course I would never bother with that, hence why I like the dremels method of doing it all in on, plus spinning fast disk are fun to play with.
If you melt the plastic cover in the cable, just file it down a little so the ferrules/end cap fits on .