Hidden next buttons

It is a really good question.

I have for years sat in the camp that you should always disable over hiding UI. 

The arguments for disabled buttons:

• If it is sequential flow where uses complete tasks in order they controls should be visible but disabled.
• It can reduce confusion as UI elements do not keep disappearing
• You can reduce the instructions as a disabled button already tells the user they need to do something
• It gives a sense of direction to the course as it pushes them forward as it unlocks 

The arguments against:

• Reduces cognitive load, users can focus more on the task and removes distractions.
• Simplicity,  saves space and allows us to change the controls at will
• Can cause confusion as users are trying to work out what the prerequisites are to unlock the next button
• Disabled controls contrast don't have to be accessible so may make them invisible
• users may not understand it is disabled and click it anyway

Sometimes it is better to test, I still go with disabled, but I have one caveat controls that never get enabled always get hidden, big.  the back button on the first slide and next on the last.