Time/effort/security/performance, really? IIRC the reasons to consider Erlang is purely to do with building a fault tolerant highly distributed system.
This. And it's ability to be introspected in realtime. It has a REPL-experience very much like lisp's do
Not sure what you mean by introspection in real time ... but if you mean runtime introspection this is not that much of a niche feature. If you mean remote debugging, again not that much of a niche feature. JVM and Javascript provides both.
I would say that at least the elixir ecosystem puts a very high priority on making sane security very easy, especially Phoenix and nerves