It may not be noticeable with smaller data sets but with very large ones (100,000- Millions of rows) you may see a degradation in performance in the refresh of charts when selections are made. It could also highlight that you have an 'ambiguous' join e.g. join on a field called Date where in fact one date may be order date and the other date is Delivery Date.
Synthetic Keys are entity that can create duplication of data, this can bring you to have a down of performances and ambigous joins.
Try to use Ctrl+T and in table viewer see how many rows your Synthetic Keys have and think that all that data are unnecessary. Then with Syn you create many connections between tables, and with large size of data you could have problem with front-end user (for examples when applying selections).
Avoid Synthetic Keys is always a best practice to follow when working with QlikView.
In my opinion (and in my job) I can't see pros in Syntethic Keys 🙂