I am not specialist in SSL, so I was choosing from most popular libraries. I found GNUSSL well documented for my (async) needs, so I've chosen it, and had no regrets so far.
Cryptography is the place where everybody is looking for weaknesses therefore the more popular library is the more weaknesses are found. Which is good by the way as all of them get fixed. So I would not reject GNUTLS just for that.
Attaching different SSL library should not be hard if it does support async model. All GNUTLS related stuff is isolated in single nxd_ssl_socket.c file.