The Problem With Single Page Applications
Like all technology, SPAs have their place. Carefully evaluate what your website really needs and what you want it to achieve. If you need a website that promotes your business or sells products or services online, in most cases you will not want an SPA site. If your website is search engine friendly, search engines will reward you with traffic. In the vast majority of cases, it's very unlikely that you need a SPA.
On the other hand, if you believe you really do require an SPA, such as an alternative to a desktop application, know the challenges that you have to overcome and build the website so that the SPA portion is post-login, with static HTML to be search engine friendly.
Finally, don't listen to the SEO companies who will try to make you believe they can SEO optimize your SPA based website. I don't want to say that SEO companies lie, but too often they exaggerate their capabilities. Not that I recommend SEO companies anyhow, but that's a topic for another article.
Be informed. Use the right tool for the job. Keep it simple.