Inserting `rel=nofollow` links inside your posts or pages via shortcode is an easy and much better method of inserting no follow attribute because here you can decide what links you want to prevent from search engine crawl.
You can configure all external links with custom settings globally in your WordPress site. For example: If you want to open all external links in a new tab then you can configure WordPress to open all external links in a new tab so that you don’t need giving blank target everytime you add an external link inside your blog post or anywhere else.
There are plugins for adding NoFollow and DoFollow attribute to all external links in your WordPress site but this is what creates problem when you want to make certain link DoFollow or NoFollow and you get messed up because of some plugin activated in your site.
Easily enforce automatic creation of links over given words across WordPress Multisite installation. WordPress plugins repository is having a simple, light and powerful plugin that allows you to do so. It works in single WordPress installations too.