Programming in assembly isn't really "hard" it mostly takes lots of discipline. Consistency and patterns are key. The language also provides very little implicit documentation, so always document which arguments are passed how and where, what registers are caller and callee saved. Of course it is also very tedious.

Now writing very optimized assembly is very hard. Because you need to break your consistency and conventions to squeeze out all the possible performance. The larger "kernel" you optimize the more pattern breaking code you need to keep in your head at a time.