And are there any limitations with multiplayer/inventory/custom UI?
Currently, there is not the ability to create static/skeletal meshes within the editor/engine. So you will have to do all your 3D modeling outside of the engine. A really awesome tool that a lot of people “building on a budget” resort to is Blender. Blender is free software that has a butt load of features. Their link can be found HERE. I would recommend searching the “TUBES” for some examples.
As far as limitations… The limitations are determined by your knowledge and willingness to build upon that knowledge! 
But as far as multiplayer, inventory and custom UI, I would recommend taking a look at some of the content and game EXAMPLES available from Epic. The answers to all your questions just may be found within!
Hope this information is somewhat helpful!!
MM
UE4 can be used on its own to create an RPG, and supports multiplayer networking. You can build your own custom data structures for inventory/items/actors, as well as your own custom HUD/UI elements.
Definitely yes. My team and I are currently creating a RPG game with the UE4 and we already have a pretty complex combat system, quest system, dialog system, inventory/item system,… everything created in the third person template with just bps and some few code lines (e.g we have connected our dialog system with a excel file so that we can easily create new dialogues.
I don’t see any content examples that are related to multiplayer/rpg
Take a look at the shooter demo
Check out the upcoming Elemental demo content.
Are you all going to make it multiplayer?