The only time that cron jobs would fail (that is directly related to cron) would be if the machine was off when a job was scheduled. In that case the cron would not run that event when you turned the box back on.

There are versions of cron that do have this feature, though. However, I would advise you to use it sparingly (think of a job that runs once a minute, and you leave your box off for a day :) ).

All other things that could cause a cron job to fail, such as low disk space, are going to be problems you will face no matter what -- and cron cannot (reasonably) do anything about them. It's up to your scripts to run correctly under those extreme circumstances.

Ciao,
Gryn