...In terms of the ability of each murder to occur.
My information is from this program: The National Geographic Program The Lost JFK Tapes: The Assassination.
https://www.youtube.com/watch?v=34K0oYGUGFkDespite what the time there says, it's half the length, as it plays twice.
This program is a compilation of news reports as they came into a local Dallas T.V station.
With JFK, once you accept the idea that the buildings and the areas around the route JFK's motorcade was travelling were not secured and that JFK's car did not have the bubble top on it, there was nothing unusual with the ability of a sniper to kill JFK from a building (or shooting from the grassy knoll if you prefer.)
The situation with Lee Oswald though is absolutely bizarre. The program mentions that the Dallas City Hall building where Oswald was kept in jail as they prepared to move him to the Dallas County Jail was secured much more the JFK motorcade route. They said that the only people allowed into the building as Oswald was being moved were police officers and journalists.
Given that the amount of police training in 1963 to secure buildings and routes was likely nowhere near what it is now, I can understand the explanation at the time that Jack Ruby was overlooked because the police said "we assumed he was with the FBI, as he was wearing their outfit."
(they may have said that they assumed he was with the Secret Service, not the FBI, I'm not great remembering names.)
However, obviously that assumes they had no idea who he was, which was contradicted later when at least several of the Dallas police officers said "we knew he was Jack Ruby because we were familiar with him through his strip club business."
So, one might think that since at least some of the police knew who he was, they figured "Well, he's just Jack Ruby, he's harmless." However, that's contradicted by the order to allow only police officers and journalists to be in the city hall building during the removal of Lee Oswald. It's hard to believe that not a single police officer there who knew who Jack Ruby was would have decided to not follow orders and have Ruby removed from the building.
Also, before the assassination of Lee Harvey Oswald, the Chief of the Dallas Police is quoted as saying "we have an air tight case against Oswald" and something like "we have no indication that anybody else was involved in the killing of JFK"
However, after Ruby killed Oswald, the news reports state that the Dallas police believed that in no way did Jack Ruby act out of 'patriotic fervor' and that the killing of Oswald was to silence him from speaking publicly. So, the Dallas Police Chief was presumably told by his officers that the case against Oswald was air tight and he acted alone, but then that Ruby killed Oswald to silence him even though Oswald acted alone.
Unmentioned in that program is that the Dallas police knew at that time that Ruby was a member of the Mafia, although I suppose it's implied in saying that the Dallas police collectively believed that Ruby killed Oswald to silence him.
That the police knew at the time that Ruby was a member of the Mafia makes it even stranger that he wasn't removed from the building as there were rumors even then that the Mafia had a role in the killing of JFK.
Edit to add: In regards to the JFK assassination, once it was decided that JFK would be riding in a convertible (that decision was apparently solely based on the weather conditions) I don't know whose decisions it was to not put the bubble top on top of the convertible (I've read from some sources that JFK personally decided that) however, if you see this video, you'll realize that the idea of securing the motorcade route is ridiculous. I don't know that every building along the route would have needed to be secured with officers posted in each building, but there were literally hundreds of buildings along the route. The only way that could have been done is if the military (or maybe the National Guard) was ordered to do this job. If for no other reason, the optics of 'JFK militarizing Dallas' would have made doing this impossible.