Problem with GenEvent.stream vs. GenEvent.add_handler

registry_test.exs

defmodule KV.RegistryTest do
  use ExUnit.Case, async: true
  alias KV.Registry, as: Reg

  # test helper module
  defmodule Forwarder do
    use GenEvent

    def handle_event(event, parent) do
      send parent, event
      {:ok, parent}
    end
  end

  setup do
    {:ok, sup} = KV.Bucket.Supervisor.start_link()
    {:ok, manager} = GenEvent.start_link()
    {:ok, registry} = Reg.start_link(manager, sup)
    
    ##### this does not work! ####
    #parent = self()
    #spawn_link fn ->
      #    for msg <- GenEvent.stream(manager) do
        #    send parent, msg
        #sometest = {:ok, :ornot}
        #end
        #end

    #### but this does! ####
    GenEvent.add_handler(manager, Forwarder, self())
    
    {:ok, registry: registry, manager: manager, sup: sup}
  end

  test "spawns buckets", %{registry: reg} do
    assert Reg.lookup(reg, "shop1") == :error
    Reg.create(reg,"shop1")
    assert {:ok, _bucket} = Reg.lookup(reg, "shop1")
  end

  test "removes bucket on crash", %{registry: reg} do
    Reg.create(reg, "shop2")
 {:ok, bucket} = Reg.lookup(reg, "shop2")

    Process.exit(bucket, :shutdown)
    assert_receive {:exit, "shop2", ^bucket}
    assert Reg.lookup(reg, "shop2") == :error
  end

  test "sends event on create and crash", %{registry: reg} do
    Reg.create(reg,"shop3")
    {:ok, bucket} = Reg.lookup(reg, "shop3")
    assert_receive {:create, "shop3", ^bucket}

    Process.exit(bucket, :shutdown)
    assert_receive {:exit, "shop3", ^bucket}
  end

end

Can anybody explain to me what the real difference between the stream and the handler approach is? When I run the tests I seem to get a race condition. Sometimes "all green", sometimes the ":create" or ":exit" messages are not received.
My "printf debugging" has shown that "GenEvent.sync_notify" seems to block indefinitely...

The rest of the source is as described in the elixir tutorial after cap 5.2