As far as I can tell they don't offer either SSL or SSH tunnelling, so there's no way you can securely connect to the database remotely. Without those you're sending your password and data unencrypted over a public network.

By default Mongo uses MD5 for password hashing (with username as seed) so it's not secure to use it over a public network.

(if your server is co-located on the same network as MongoHQ then you can probably hack it using network access rules)