I'll start with JustMail since that's the shortest answer. I requested but did not receive an invite, and none of the contacts (Eric,Vladmir, Harley) with email addresses on their About page are working. They're locked down or don't exist on LinkedIn. I'm unable to evaluate JustMail and provide comments specific to their service.
In addition to being decentralized at least some of these services send encrypted instead of plain text email.
A pro for decentralized email, depending on your perspective, is that there shouldn't be some core group of servers that is a point where a subpoena can be used by authorities to hook in and get a copy of users emails. These services may offer an extra layer of protection if concealing your identity is a top requirement for your email hosting.