NES games were still pretty simple, so I can see how simple tools were more or less adequate.
I'm far more impressed that complex SNES games were also developed purely in assembly, like A Link to the Past (1991) or Super Metroid (1994). This is an era where PC game developers would mostly be using C.
You may be interested in this series on SNES hardware features and how they were exploited. The videos have very informative visual representations of memory/io/etc. (the other videos on the channel are also good)
On the other hand, there were pretty complex games on the vastly more simple NES. Less than on the SNES, they required elaborate tricks. (I guess that however mostly required more from the actual game code, and sometimes some extra hardware, than from the tools.)
I'm far more impressed that complex SNES games were also developed purely in assembly, like A Link to the Past (1991) or Super Metroid (1994). This is an era where PC game developers would mostly be using C.