The only thing I can hear that sounds unnatural, is the way that the reverberation in the room (the "echo") immediately gets lower when the raw piano sound itself gets lower. In a real room, if you produce a loud sound and immediately after a soft sound, the reverberation of the loud sound remains. But since this network only models "the sound right now", the volume of the reverberation follows the volume of the piano sound.
To my ears, this is most prevalent in the last example, which starts out loud and gradually becomes softer. It sounds a bit like they are cross-fading between multiple recordings.
Regardless, the piano sounds completely natural to me, I don't hear any artifacts or sounds that a real piano wouldn't make. Amazing!
There are also fragments that sounds inspiring and very musical to my ears, such as the melody and chord progression after 00:08 in the first example.