You have to stay organized to remember where all the parts go when you put it back together, but it's not unduly difficult. You do have to make sure the gears are aligned ("timed" in the manual) properly so that the teeth on the planet gears face radially outward at once. The manual has a little diagram of that.
I rebuilt and oiled this hub this afternoon. Again, organization is the key, and have a copy of the manual with you when you rebuild the hub. But otherwise, it's just a small step up in terms of difficulty from the AW, I think. It's not a Swiss watch, but you have to be careful taking it apart and putting it together is all.
What do I do when I need to rebuild a Sturmey hub?
- Disassemble and clean: WD-40 or Kano Kroil and Acetone, a rag, and canned air to clean out the hub parts of old oil and grease.
- De-burr parts as needed (use of a needle file and fine sand paper). Some people don't do the de-burr, and often it isn't needed. Every so often you find little spots that need to be smoothed a bit; I knock down any burrs as needed.
- Replace anything that's broken (usually the parts are OK - vintage Sturmey parts usually have a long lifespan).
- Re-oil internal contact points, put new grease seals in the outer bearings, and reassemble the hub. Check bearings for adjustment and put back on the bike.
Manuals for the S5: