Hash is so powerful in Ruby that people often overuse them.

One of the most common issues I found on Ruby code-bases is to not create classes to represent their domain and simply use hashes everywhere.

The downside is that a hash has no shape. It can (and will) be anything you want it to be, often causing havoc once the system grows.

Checks for keys everywhere. Almost all statements use the safe navigation because you never know what shape you're dealing with. Multiple places performing the same map/reduce/filter/etc. All because people stick to hashes a bit too long.