Kontakt 5.6 contains a major UI refresh, which brings it in line with other software and to current market UI trends.
We have had reports of issues with stability and registration from new users as well as updating users.
An option to show the UI in a larger font size has been removed, making Kontakt much harder to read on high-res screens.
We have also had reports of some settings not correctly being applied in 5.6.
All Orchestral Tools collections work fine in the previous version, Kontakt 5.5.2 (you can see the minimum requirements at Current Versions).
There is no technical reason to use Kontakt 5.6, especially if you need the "lost features" mentioned above or are wary of tech issues :)
All Collections should generally work fine in 5.6, if you want to update, but at present we recommend to stick with 5.5.2 until there is more information about the features lost, etc.
For this reason, our documentation will continue showing Kontakt 5.5.2 for the foreseeable future.