Custom validation help

Since ‘create_new_user’ uses ‘register_post’ it validates the registration form before ‘registration_errors’ fires. This only permits custom validation to work if the username or email has errors. If they don’t, the form will ignore any function with the hook ‘registration_errors’. The WordPress website states to never validate forms using ‘register_post’. So, my question is how do I add custom validation? Should i just add to to the core functions of the plugin like so:

public function create_new_user( $user_login, $user_email, $errors ) {
		$errors->add( 'demo_error', __( '<strong>ERROR</strong>: This is a demo error.', 'my_textdomain' ) );
		if ( $errors->get_error_code() ) {
			return;
		}

		// create the user
		$user_pass = wp_generate_password( 12, false );
		$user_id = wp_create_user( $user_login, $user_pass, $user_email );
		if ( !$user_id ) {
			$errors->add( 'registerfail', sprintf( __( '<strong>ERROR</strong>: Couldn’t register you... please contact the <a href="mailto:%s">webmaster</a> !' ), get_option( 'admin_email' ) ) );
		}
	}

What are my options. Is this code really unsafe like the WordPress site states?

https://wordpress.org/plugins/new-user-approve/