I've noticed a feature that seems to be have been lost when we switched from neon to serf: serf doesn't store the client cert creds. There are two bits of data, the path to the pkcs12 file and the password for the private key. The ra_serf library will prompt for these, if config:auth:ssl-client-cert-file-prompt is set, but there is no code to call svn_auth_save_credentials() and they do not get stored. The ra_neon library did store them and all the relevant code is still present in the providers.
I can't see any reason for dropping this feature so I think it should be reinstated. -- Philip