Is it safe to chmod 777 everything?

By changing everything to 777, you are bypassing most of Linuxes privileges security settings, and this would be considered unsafe, and definitely not best practice.

The problem is that if someone can penetrate your system as a user other than yourself, if your files are CHMOD 777, they can read, write, execute and delete them. This is considered a security threat.

Similarly, having files with "777" permissions means that if someone can find a non-executable file, and overwrite it with a script, and can trick you to run that file, it will execute, which is a security risk.

If it were me, I definitely wouldn't do it, but if you are the only user of your system, and you are not worried about targeted attacks, you might be able to get away with it on an Android device (you are vulnerable, but somewhat insulated by the privileges system and single-user nature of the device) - but doing it on any kind of Linux server is begging for it to be hacked.

There are many situations where chmod 777 actually breaks functionality. There is absolutely no situation where you want to do this.