Discussion board where members can get started with Qlik Sense.
Using extensions in QlikView or Qliksense it depends on your requirement that what exactly you are trying to implement.
If you are trying to achieve any custom visualization that are not available within qlik then you can go for extension objects.
take a look at Certified Extensions - Coming to a Cloud Near You!
I know extensions sometimes could be useful for UI purposes when you need some viz customization, however if the are not certified you could face some errors after software upgrading.
As agigliotti very rightly mentioned, the risk with extensions is that you are yourself responsible to support them which requires advanced knowledge in Qlik API/Web Development technologies. Also, given the pace of new Qlik Sense releases (5x a year) this might become quite a hassle if you are planning to upgrade often (e.g. Feb 18' introduced some interesting changes in the qHyperCube structure which may affect extensions).
If this is something you do not wish to worry about and you still want to leverage extensions for added functionality in Sense, I recommend going for a fully supported extension library, like e.g. https://www.vizlib.com