It's probably a lot easier in a way, because they don't have to worry about external dependencies changing at all. Modern code is a real PITA that way.
What's hard about their work is 1) it's really, really slow to communicate with, so you can't iterate quickly, 2) the tech is really old and unlike today's stuff, so it's very specialized domain knowledge.
The lead engineer from the 70's is actually in Voyager 1's fuselage. It's cramped but they seem to be doing ok! They're just taking pictures with an old Canon point and shoot.
What's hard about their work is 1) it's really, really slow to communicate with, so you can't iterate quickly, 2) the tech is really old and unlike today's stuff, so it's very specialized domain knowledge.