Hey Karom,
Without a doubt, Disqus is a great comment tool and it's really easy to integrate and manage. If you look at their FAQ section, they claims that, it is SEO friendly (https://help.disqus.com/customer/portal/articles/1233994-common-questions-about-disqus).
In the past, Disqus was not friendly and you had to use Disqus API in order to get your comments indexed. But now, they have worked on this issue and it seems the comments are now easily indexed even if you're using the simple Disqus program.
For the prove, please look at the cached copy of this post:
http://webcache.googleusercontent.com/search?q=cache%3Abuildfire.com%2Femail-marketing-tools%2F&ie=utf-8&oe=utf-8