You do need to look after a steel bike more because of rust. Basically if you want to keep your steel bike in good condition you will need to keep touch up paint handy for scratches etc and you really need to "drain" your bike after riding in heavy rain, especially the seat tube because these will rust out.

You can for extra cost get the inside of the frame's tubes coated so they won't rust.

The only time I have seen carbon crack is because of a bad accident (and any bike then will have damage), a pro level cyclist putting too much power into a sprint or someone over tighteneing a clamp and this can be avoided by using a torque wrench to tighten things.