I'm a machinist and have used Socket Head Cap Screws (Allen bolts) for 50+ years. In my world there are only black oxide coated, steel bolts. For anything else I would add SS or Nylon and know that they are weak. Flat head and button head are also made to a different spec and are weaker and not in consideration.
If you look at the chart on bolt strength, there is only one spec for all SHCS, both min proof and min tensile strength. Both higher than grade 8 bolts. So, when any SHCS is manufactured, it has to meet those requirements, same as grade 8 bolts.
That is what I was going by and have always thought of them as quality fasteners and I would not hesitate using them as head bolts but, as rod bolts I'll be diligent if, I were to buy these. The reason I like them, is because of their design and I can see a strength advantage in the serrations,over dowels but, the offseting thing, is their age and unknown history. I suppose I could Xray and mag them.
I can remember that even using grade 8 bolts for bell housing bolts wasn't recommended, in the '60's and '70's because of their low elongation percentage.