Optimizely is in the process of putting a CDN in place for all Spire Sites and Classic sites that have migrated to containers.
CDNs are great to improve site speed, however, best practice would be to implement the CDN as a subdomain of the main site. For example: cdn.YOURDOMAIN.com
When turned on, our CDN currently shows all images and documents hosted on 'a random string of characters' on the cloudfront.net domain. Example: https://d3mvezagf9plvn.cloudfront.net/
From a UX prespective, if your sales rep or customer shares a link to a spec sheet PDF from your site, it will look like a spam link, which saavy users (and email providers) have been trained to look for in emails to prevent phishing attacks. Example: https://d3mvezagf9plvn.cloudfront.net/userfiles/documents/dixon_shc0562_catalog.pdf
According to an article from SEMRush, "Enabling CDN using your own subdomain keeps the service proprietary to you and points SEO values of the CDN based resources to your root domain — yoursite.com instead of cdn.com. This approach maintains or even increases your main site's authority scores. This in turn boosts your SEO rankings. And of course, you are not bypassing your domain value to a third-party CDN site while improving page speed."
As currently implemented, using the cloudfront.net domain, users will lose SEO value. Images and documents no longer appear to be hosted on your site, negatively affecting your brand authority, and removing important signals for SEO.