I've been playing a lot of Vault of the Void, which is honestly mostly a mouse-only game on the Deck, but being able to map a few key keyboard shortcuts to the plethora of buttons makes it feel effectively the same.
Honestly, its more that it feels like a new input type entirely in a class of its own. I find I use it a lot to mix and match gamepad controls with keyboard shortcuts and mouse click-drag, sometimes all within the same game.
Noita has gamepad controls, but a lot of the interface works a lot better with mouse+keyboard. Deck lets me use gamepad controls for when my character is interacting with the environment, and use mouse+keyboard controls when I open my inventory. This wouldn't be realistic on another device, because the environment/world continues in the background even while you have your inventory open, so you need to be able to switch immediately from inventory management back to world-interaction.
Most games are like this: they rely heavily on one input, but being able to make use of the other inputs simultaneously gives you a ton of options.
The one class of game that I really enjoy but haven't tried yet is heavily-keyboard-focused games like some traditional roguelikes. Some, like Brogue, have a small enough control set that it would be pretty easy to map all the main keys, and supplement with mouse input for eg inventory selection. Many require more keys and don't allow mouse input, however. Cataclysm: Dark Days Ahead has something like FIFO keyboard shortcut for inventory items, and uses a-z plus several symbols; that would be pretty painful on Deck. I haven't played that in years though, so maybe would be better today. I've been seriously meaning to try Cogmind or Caves of Qud on the Deck, though.