While the Famicom was decent, the NES had mechanical problems.
The Famicom was a top-loading console, like the SNES, but had hard-wired controllers that came out of the back of the console. There was a controller port on the front that was used for the light gun peripheral.
The NES had pluggable controllers, but the cartridge was front-loaded. The cartridge would plug into the bus in the back, then the whole thing was pushed down, which allowed the contacts on the bus to touch the contacts on the board.
Those contacts often became worn and lost their elasticity over many uses, basically becoming more curled over time. These would then not make full contact with the board, causing games to not load.
They eventually redesigned the system to be a top-loader like the original Famicom. They also introduced this design to Japan so they would gain swappable controllers.
Also, the Famicom cartridges were smaller, just large enough to contain the board. NES cartridges were much larger with a lot of empty space. Didn't cause any issues technially, but it was a choice.