Erlang has weaknesses, but certainly those are not it. Basically, Erlang excells at writing servers. Take a look at YAWS (http://yaws.hyber.org/) for a web server written in Erlang that can actually outperform more traditional web servers in very realistic scenarios.
If I had to name a weakness or two of erlang offhand:
1. Integrating foreign code with Erlang is very different from traditional FFI and requires some initial legwork (libraries like Erlectricity http://github.com/KirinDave/erlectricity/tree/master , for example).
2. String manipulation. Erlang doesn't have a real string type. Its string libraries and core string type assume using linked lists for strings, which is spectacularly inefficient. However, there is a movement and a proposal to change the string type to Erlang binaries and provide an alternative string library. Hopefully that will address the problem.
Other than that, it's a reasonably fast and rock-solid implementation of an FP language with a mountain of solid IPC library code integrated at the language level.