The answer is both simple and complicated at the same time. Simply put, the ATF is not consistent in the decisions and rulings it makes. As a "general rule", and not in any way, shape, or form meant to be legal advice, the ATF generally looks at the intent of what a firearm or sear conversion what originally meant for when it was originally registered.
In the example of your HK sear, the sear/trigger pack was originally designed to be installed in multiple HK firearms and clones, so the ATF allows it.
With an AR-15 DIAS, it gets more complicated. Assuming you have only one DIAS, you can only have one lower with a FA trigger/hammer group, as the ATF sees these as conversion parts. You can, however, have multiple uppers with FA bolt carriers (the ATF has explicitly ruled that FA bolt carriers are ok in SA rifles). It is also perfectly ok to mill a high shelf AR-15 lower into a low shelf in order to use a DIAS, and this is not considered to be manufacturing a machine gun. Technically you can move the DIAS from lower-to-lower, but since you can only have one FA trigger/hammer group, this can be more complicated. For this reason, I know several folks who have bought MGI lowers w/ their modular mag wells so that they can gave one "DIAS lower", but easily convert it to multiple calibers for different uppers quickly.
In the case of drilling a hole for a GI auto sear in an AR-15, there is no such thing as a transferable GI sear. This means that the only reason for ever drilling a sear hole in an AR-15 lower is if you are intending to make a new machine gun. Now were back to the 1968 GCA definition of a machine gun any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun
FNC sears are different than both AR-15 DIAS's and HK sears/trigger packs in that the FNC rifle requires machining, welding, tooling, and extra parts (pin, sear spring, etc) to add the registered S&H sear. There is nothing "drop in" about it, unlike the DIAS and HK sears. Also, the FNC sears were specifically designed and registered with one intent in mind; converting a FNH FNC rifle to legal full auto operation. Unlike the DIAS and HK sears, they were not designed to be easily moved to different firearms within the same family. This is why the ATF takes a narrow view on the S&H sears and has only approved their use in FNH FNC rifles to this point. Being who they are, they can always change their mind and allow the sears to be installed in other "similar" platforms, but I doubt it. FNH themselves has already stated that the SCAR is a whole new platform and not based on any previous design. I.E. you can't claim it to be an "improved FNC" or "FNC type rifle" because the MFG says that it isn't.
All my own views as a Class II MFG who's family has been in the FFL game for over 40 years.