I approach risk management for the 12 year recommendation differently than I might for operating TBO. The limiting cases are so much wider apart: if the engine was stored in cool dehumidified air or dry nitrogen it might be as good after 12 years as it was on day 1. Conversely, if it is stored outside in a very humid environment it could corrode fast. And where the engine sits within that spectrum is important, because you take a risk disassembling and bolting together a perfectly good engine - mistakes can be made, and parts can be damaged.
I know a guy with two newly overhauled radial engines in storage, waiting for use on his biplane someday. They are stored in cooled, dehumidified air 24 hrs/day. He bought them because the expert who built them wasn't going to be around forever and he was apparently the best with these engines.
The 40 year old original build Lycoming I mentioned above has mid-time operating hours, and I know where it was stored (and by whom) for most of that time... It doesn't leak a drop or burn much oil, and it was inspected pretty closely when I bought it. It has a newish carb and mags. That doesn't reduce the risk of operating it to zero, but the risk is balanced against the risk of overhaul, as well as the cost. For now I'm happy to fly it, and "for now" could last for years (or not, if things change).
I do find the orange pushrod tube seals on that engine curious... on other Lycomings I've seen they are always green. Maybe this is because I haven't seen many that were installed in 1971!