I am using MySql for my backend. I have run into a few issues with because of this.
1. The logging needs to be disabled (unless you are able to update the app_data folder). The site will still work properly, but the logging won't work.
2. You have to turn off recaptcha logging. The commenters will receive an error, but the comment will be posted. This is due to the call BlogService.LoadFromDataStore. The Xml provider stores the data as a stream, the DbBlogProviders
store the data as a string. If the logging is enable users will get the "Call back has failed" error message. If you have custom errors turned off, then they will get the "Unable to cast type System.String to System.Stream".
I have only used the blogengine for a few weeks, but this is what has occurred so far. I do like the engine as it has made it so easy to create the blogs and avoid spam.