The Steam Deck is probably as much plug in and play as it gets with being a PC. However, it's still a PC so sometimes you have to tinker a bit to get that one specific game from 1999 to run or the other one from 2022 that may not work on Valve's official Proton version.I like gaming without much complication,
Thousands of games work out of the box, though. You'll still encounter minor oddities at times, e. g. when navigating a huge library and the cursor goes wild.
However, Valve is constantly pushing out minor and major updates, further improving on features, stability, and performance.
As for the games, even RTS games work well as long as you don't play competitive. Games without gamepad support can be played with the Steam Deck but require pre-made community controls or tinkering with SteamInput on your own. Once its setup correctly it does work well; trial and error is part of the process though.
If the game supports controller natively, you're good to go in almost all instances.
The speakers are exceptionally good and loud if you want to and retro games are amazing on that display and with the built in speakers.
Personally, I wouldn't want to play games like Elden Ring or Cyberpunk on it. Many do though and that's the brilliance of it. Gives you lots of options but I think it works best when playing older games that won't make the fan start, gives you 3-6h hours battery life and offer native game pad support. For those it's like playing on a console.
If you feel adventurous you can always do more with it. I'd also recommend it highly for pre PS2 era emulation.