In my experience it isn't almost free. The "problem" being the overloaded nature of processes: their sole purpose isn't to isolate errors, its also to provide concurrency, a sort of access control (since a process goes through its mailbox 1 message at a time), and you also have to consider data access performance characteristics (since sending data across processes is copied).

So in an ideal world yes, you can isolate them, but I've never really achieved this panacea in practice, and its never been anywhere near free to try to suss out a design that optimally achieves all those differing design aspects at once.