[le-mark]

    No runtime
    No BCL
    No objects or GC
    No debugger
    Strings are ASCII

I did not see the readme mention anything about bool/byte/char/short/int/long sizes or signed and unsigned. It can be quite a surprise to find yourself with 8 bit long longs! As some found with sdcc for gameboy.

[noobermin]

The NES has 2K ram, where you can get carts with mappers that allow you to use prg as ram as well though, this is still limited. Also, the "stack" as it exists must fit on a page (256 bytes). Putting a runtime or sorts onto an NES itself is a big feat given this and requires a lot of manual twiddling.

[bagful]

The hardware stack pointer is restricted to indexing within [$0100, $01ff] but, lacking a stack-pointer-relative addressing mode, it is more preferable to realize a parameter stack using a zero-page pointer indexed with the (zp),Y addressing mode, as cc65 does.