Traffic analysis is based on who is talking to who, as opposed to the content of messages.
Who is talking to whom tells reams by itself. There are three different types of remailers. Just leaning the differences between these types will tell you most of what you need to know about remailers, and what the security issues are. I only know of one Type 0 remailer. It was operated by Johann Helsingius in Finland at anon.penet.fi until it got zorched by Scientologists for alleged being used to distribute classified church materials.