I'm not really sure that's true; he seems to have gone to a fair amount of trouble to make sure that the expanded form is readable, at which point debugging it

I'm not really sure that's true; he seems to have gone to a fair amount of trouble to make sure that the expanded form is readable, at which point debugging it should be no trickier than debugging any other macro expansion - just keep expanding step by step until you find the thing that went wrong. If anything I'd expect this to be easier to deal with than multiple layers of plain C macros given it's going to be a lot easier to introspect - at least for the same level of complexity of expanded result. Of course, if you do substantially more complex things with it then, yes, it's going to be more complex to debug. But that's always the price paid for using complex abstractions - complexity can be corralled, compartmentalised and contained but rarely eliminated entirely.