My group had fun playing it for a few months with just the 3 core books. Also, it does a lot that makes combat varieted and fun, never does an experienced Fighter “just walk and attack” for a turn in that game! I’d say it works at least “pretty well” on its own

But it leaves little room for player interpretation and for them to come up with their own solutions; even something like “I shoot arrows into the wall to help us climb out of here” is a Power, not something you can just decide to do…