It's worth noting that Elixir (by descent from Erlang) can emulate at least a basic static typing system pretty easily through pattern matching. You miss out on some features common in more traditionally-object-oriented languages (namely: subtypes), but tagged tuples and structs do provide a lot of the same safety benefits in runtime (and tools like Dialyzer can - last I check - use such pattern matching as a basis for static verification).

Elixir and Erlang still lack the ability to typecheck any process behaviors because messages can take any type anywhere.

You can still pattern match when receiving, though (in fact, 'receive' in both Erlang and Elixir does pattern matching already in the same vein as 'case'); just match messages based on a tagged tuple or a record (Erlang) / struct (Elixir) signature or however else you want to define your "types".