Security issues, as in any project can change the value of the database; not privacy issues. But that's something you should be worried about as well. What if someone sends personal info through cloud variables? It's not hard to turn text into numbers and back.
Again, these aren't cloud variables. No backend is storing the data.
Sure, you are sending the received data from the MQTT broker to the browser's local storage; but what if no one is online to send/receive the stored values from one PC to another? The data will never be kept in circulation. In case you didn't know: the database library doesn't store any data in the project itself; it stores it to the hard drive of the computer. No one else can access these values from another computer without sending the value over the internet. For these to be called cloud variables, you need to make a secure cloud variable server that Snap! can access at any time, regardless if anyone is using the project or not. Nice idea, but it just won't work.