Sharepoints are a convenience and don't necessarily convey any extra security, and they can bag you in various ways.
There have been cases of protocol-level security exposures in AFP; one was patched a few months back.
Further, file-based services can have security issues around what can be uploaded (and unrestricted file-type uploads or gifar-style upload attacks can be particularly nasty), and there are some rather nasty attacks that can be deployed here.
As for Open Directory, that can be useful for clients that are joined to the domain, but I'm interpreting "client" in the OPs question as a business client and not a "client system", and passing out usernames and passwords implies they're not members of the domain.
Unrestricted (either intentionally unrestricted, or due to insecurities or configuration errors) file upload services are regularly sought after by attackers, and the servers can then be filled with warz or other nasty stuff.
And the most common transport protocol is ftp, and ftp is hideous around firewalls. It's also insecure; the credentials are cleartext. You might as well post the login credentials directly on your web site. (What can also happen here is a case of accountability and of password reuse; the same username and password gets passed around and embedded everywhere with many of these ftp servers, which often effectively means the password is in the wind...
Website-based uploads can mean the remote user doesn't necessarily need a login (and particularly if they have no way to access nor reference nor guess the (renamed) name of the uploaded file after they upload it) and website-based downloads mean the remote user only need be provided with a temporary and mostly-random URL to fetch the file, and the package can then clean the file off automatically.
Sure, sftp and ftp do work here. (And in addition to not exposing credentials as does ftp, sftp is way easier to deal with around firewalls, and you can use individualized and revokable certificates, if you so choose.) These schemes just (in my experience) turn into a mess. And a directory filled with dreck, if somebody's not actively watching it, unfortunately. And no, I don't prefer to expose SMB/CIFS nor AFP nor NFS to the Internet wild-lands, either.