Calling bundle update
(without arguments) updates all your gems at once. Given that many gems don't care about stable APIs, this might break your application in a million ways.
To stay sane, update your gems using the applicable way below:
Update the entire bundle regularily (e.g. once a week). This ensures that your libraries are up-to-date while it's easy to spot major version bumps which may break the app.
It's also advisable to only update gems for a good reason (security fixes, new features you want), unless you like pain.