Full of bad chemicals, you need to read the code stamped on them for which nasties they contain plus splinters as rough sawn shitty softwood.
Forget hand sanding them.
Face-ache marketplace or similar for a proper bed base or at leased some planed timber from diy store and buy the slats from ikea.
Pallet beds. Yes. I am tight.
They’re pretty simple right? 4 pallets, sanding, brackets to join them together.