I upgraded my Steam Deck with a 2 Tb NVMe and I've put over 200 games on it. Most games even the newest resource hogs run well on it. I prefer playing any games that draw less than 15 watts as the fan doesn't kick on and the deck isn't working as hard. The last two days I've been playing Nier Automata which runs amazingly well. Some other games I've played are: The Forest, Enter the Gungeon, Fallout 4, Grand Theft Auto 5, Star Wars Battlefront 2 and they all run great.
You can install emulators, PS3, Switch etc. it all works.
The controls are responsive and perfect between the analogs, touchscreen and haptics there's always an option that's comfortable. The customizability is impressive too; one example is I mapped one of the back paddles to magnifier in case there's any microscopic text (which is rare).
Yeah, it's designed to be upgraded so the process is fairly easy.