As mentioned above pallets vary widely. You have your soft wood pallets, hard wood pallets, pallets with hardwood runners but softwood slats, pallets with 4 runners vs. 3 runners, 4 way pallets (forklift can pickup the pallet from all 4 sides), and 2 way pallets. So it will vary a lot how easy it is to take them apart.
If you don't want a super rough look then I'd recommend planing the wood rather than sanding, but that could also take a while if you don't have a power planer