Kontakt 5.6 and later contain 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.
Another new issue is that selected outputs are correctly assigned, but the UI does not reflect which outputs are assigned inside patches (for example in the Mixer). So everything works, but it is not possible to see the assignments after they have been made.
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. and later
All Orchestral Tools collections but Berlin Orchestra Inspire and Metropolis Ark 3work 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 or later, especially if you need the "lost features" mentioned above or are wary of tech issues :)
All Collections should generally work fine in 5.6 or later, 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, we will continue using and recommending Kontakt 5.5.2 for the foreseeable future.