Deck is very DIY friendly, including potential repairs. So it should be fairly safe to assume that it work just as new unit - especially coming from Valve itself.
They have to say not to open so that they aren't liable for people breaking it when opening it. Some aspects of the hardware are hard to work with, like replacing the battery, but joysticks and screens (the main things I would expect people to need to repair) are surprisingly easy to access for a modern portable device.
After cracking open my own Deck, replacing the SSD or Thumb sticks is dirt simple. There are even drop in Hall effect sticks you can get. The only real trouble is if you need to replace the battery. The screen and battery are definitely the hardest things to replace in the Steam Deck.
PS: REMOVE THE MICROSD FIRST! I've seen people forget their card is in there and literally snap it in half when opening up their Steam Deck.
The only thing you really need to worry about is the metal heat shield removal since it is moved before the battery is unplugged, but after that it's pretty safe