The Nvidia Shield ran Android. So either bad Android games locally, os streaming. The Shield might do better today as a client just for Stadia, Xcloud or PS Now.
However, this is far better. Not only are the specs very impressive for the size/power, but it runs an OS that can plan anything (with Proton). It's like mixing the switch with a ps4. Imagine playing the Witcher 3, or Doom Eternal on this compared to the 480p blurry Switch version. I hope this does well.
One major difference (besides the form-factor obviously) is that the Deck doesn't require WiFi + desktop to play your PC games. I'd also argue that the trackpads will be a major difference. The Steam Controller's trackpad wasn't perfect, but it made possible to sanely play a ton of designed-for-mouse games