This looks like something a really educated/disciplined company could use for creating some crazy cool infrastructure. It's hard to see it taking off for the average Joe.
An anecdote I use when describing the benefits is the story of a smart fridge that got hacked and became part of a botnet sending spam emails. Why did that fridge even have code that allowed it to send email? It wasn't necessary for its functioning. We should write software differently if were going to be deploying it to 10x the number of devices compared to today.
More personally, I worry about the software that's going to find it's way into the embedded health devices of the future (cf pacemakers). These devices will inevitably be 'connected' and I want to make sure that the code they use is safe and secure.
If it has a remote code execution vulnerability, it's trivial to make it send spam (or do all kinds of things) whether a MUA was already present or not.
It's much easier to reason about a bunch of OCaml code, so say the authors, than it is to understand the interaction between independently changing pieces of your "stack" that are written in different languages and integrate in disparate ways.