Problems is not the right word,
constraints is the right term. The engine is limited in its capability to create an outdoor map. Outdoor environments require much more realism and clever use of The Dark Descent's original or Machine For Pigs outdoor assets. Furthermore, making use of heavy particle systems such as rain or a lot of fogs within the player's perspective can really hurt Amnesia's processing capabilities.
HPL2 was never really built for outdoor environments either, as there are two moments where you are outside in The Dark Descent (Study(?) and Zimmerman flashback) and both don't use a skybox and limited exposure to the actual outdoor environment.
However, when you know what you're doing, you can definitely make some amazing work.
For example, CarnivorousJelly, a goddess of mapping, put this together:
Or this other lovely piece of work by echohalls
Don't be fazed by the information you read until you try it for yourself!