My Steam Deck arrived about 3 weeks ago and I've been having a hell of a time with it!!!!
It runs MK11 at a full 60 fps at max settings, but I actually turned most of those settings down, along with the TDP in order to keep the system cooler and extend battery life. I was even able to play online once I changed the Proton compatibility layer to Proton GE 7-16! The D-Pad is pretty damn decent and is perfectly fine for Mortal Kombat. I don't get any erroneous inputs at all, even though it's glossy. I will admit that it's harder to do iAFs with Kitana in MK9, but I can pull off all my combos just fine offline and online.
MK9 looks absolutely gorgeous on this device too. Haven't installed MKX yet, but there's no reason to presume that it won't run flawlessly on this. Not to mention, all my other Steam/Epic games, like WWE 2K22, Shadow of The Tomb Raider, and Sifu all run at a near constant 60 fps, with Batman Arkham Knight running mostly between 40-60 fps depending on the scene.
However, the biggest strength of this device is its emulation power. I got EmuDeck running on mine and it plays Switch, Wii U, Wii/GameCube and PS2 without trouble. The Switch emulator, Yuzu does crash after a while, but the rest of the emulators do not. If you want a single platform to play all your old titles on it, the Steam Deck will more than suffice.