It's not very safe because it requires Java installed on your machine to work.
What??
Having Java enabled in your web browser may not be safe, but there is absolutely nothing wrong with having Java installed on your PC.
Dangers occur when there are vulnerabilities discovered in Java (happens too often...),
and you or your browser runs a malicious app. There is no reason to believe MutliBit is a malicious app, so as long as you disable Java in the browser (which in Windows can be done easily via the Java Control Panel app) you are safe from these types of attacks.
Maybe there are other reasons not to like MultiBit, but the fact that Java
in the browser might not be safe? That isn't a reason....