As far as data goes, this is pretty confidential stuff: your clients, bills, basically all of your business information, so it needs to be handled with care.
No one but you can access your account.
When you create your account, your password is encrypted, which means that even someone with access to the database would not be able to figure it out.
The application is based on a proven, well-tested framework.
Nothing is 100% secure, but lots of measures have been taken to keep your information secure, and any vulnerabilities that are detected are patched.
No one will ever access your data, except in the following cases:
There are technical problems, in which case, access will be limited to the parts of the database that are causing the problems.
Required by law enforcement or the courts.
Aside from these two cases, no one but you will have access to your data.
Billing and document information is just about the most important information a translator handles. Losing it is not an option (and my data is in there, too).
When it comes down to it, you are placing your trust in the application and in the people who run it. If you have any questions or doubts, feel free to get in touch with me.