One implication is that it seems to be getting easier to isolate the serum - Erskine needed to bombard it with Vitarays, Isaiah Bradleys serum killed most of the test subjects, Banner triggered something with major side effects using Gamma rays, HYDRA required a slow drip but Nargles working in a container got it to work via a quick shot/swig with no obvious side effects.
Unless Nargle was some superior genius who is to say who the next person will be able to isolate and use...
However, when he returned after the Blip and found that the project had been disbanded, there's no indication that he needed to recover any samples before he went to work for the Power Broker and produced his working serum within weeks. Which suggests that he'd already gone beyond just rebuilding previous versions of the formula, and was able, essentially, to create it from memory with appropriate raw materials.We should separate between how easy it is to isolate from refinement of it once you've got it.
Nagel says he was recruited by HYRDA for the Winter Soldier program. Then, when HYDRA largely dissolved in 2014 as a result of events in The Winter Soldier, he was recruited by the CIA. Only then did he get access to Isaiah's blood. So, we are talking years of groundwork + access to samples, which doesn't exactly sound "easy".
Which suggests that he'd already gone beyond just rebuilding previous versions of the formula, and was able, essentially, to create it from memory with appropriate raw materials.
That does suggest that his self-description as the new Erskine wasn't entirely an idle boast.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.