As far as the security of your data, this seems to always be a balance between security and convenience, which is why we still offer the local WiFi sync methods for people who don't want to use the cloud.

Here's a link to security information on the Dropbox site: Is Dropbox safe to use?

In addition to that, the Cloud Wallet file format (extension .cwlt) contains a normal wallet file (.wlt file) that is compressed for faster, less expensive data transmission and encrypted for enhanced security.