How could early reviews say this game runs at a steady 60fps? Seems crazy. I'll agree it runs at 60fps most of the time, but the game definitely dips quite a bit.
I don't know how comprehensive these reviews are. From what it looks like, publications had about two days of lead time ... maybe less than a week or likely didn't get review code at all.
Performance depends on how complex the world is. If you load a flat environment and want to build structures, you're likely going to be smooth sailing at 60fps. The creative and inventory menu systems seem to have a strange amount of lag. The UI speed setting in the options menu doesn't seem to improve how the cursor moves around with a lot of items on display or in creative mode menu.
It doesn't kill the experience but from what was said Time interview with the developer about the game being unable to switch seamlessly to different resolutions from portable <-> TV mode, sounds like the game wasn't optimized and ran satisfactory enough to get out the gate.
I suspect if/when they get a handle on the hardware, performance will improve. Bomberman and Zelda are two examples that had patches that improved the performance of the game.