What about using the devise gem? Is it safer to create our own user authentication than using a gem like devise?

Devise is the most commonly used authentication system and you will probably eventually use it in most projects. Gems like Devise were created to save time when doing things that are very common and seen in almost any project (imagine writing your own authentication EVERY single time). It's good to know how it works under the hood, so this course should be done at least once before tackling Devise. The gem is similar to what is done in these videos, and also has some additional features and helpers that you may not need. It's definitely safe (safety level is probably the same, since Devise also uses bcrypt), but customization and configuration is somewhat harder, since lots of stuff is hidden behind the gem and you don't access it the same way you would access your own authentication system.