The flexibility of building complex apps with Java/JavaScript was an extremely positive point about this tool! It's not so much that I have problems finding bugs - it does happen sometimes but usually they are pretty easy fixes once discovered (or reported!).
There seems some confusion over how long things take before being processed which can be frustrating at times if you're expecting something immediately after submitting your code online via IDE console etc.. That said though there doesn't seem too many other issues when using JSFiddle as part web server environment itself works quite well also supports integration into Google Cloud Platform services such AWS Lambda functions amongst others.