There is NO differences between qlikview and qlik sense in terms of synthetic keys implications. And it is neither good nor bad; all depends on how you have created your data model. There is classic discussion thread on this topic here: https://community.qlik.com/t5/QlikView-App-Development/Should-We-Stop-Worrying-and-Love-the-Syntheti... you can go through to understand it better.
In nutshell, if you know the impact of synthetic keys (they are just like composite keys) and you created it in your data model knowingly - it's okay; and if you don't understand the impact of it and it just has come in your data model because you could not handle it - you should be worried about it and probably try to remove them from the data model.
