I use ASEM and ASM51 for years, it is a free 8051 ASM compiler,
    no bugs reported yet AFIK, even with macros and else.

Assuming that you consider an assembler a compiler.


    some of this source files counting more than 20 thousand text
    lines (they don't fit in the regular 640k DOS memory, needing to
    split them in several include files).

Hey!  That sounds like a bug to me!  Why should a source file have to fit
in memory?


    The same for Microsoft 8086 ASM.EXE compiler, using for many years
    (probably more than 14), never hit a hole in the cheese.

I vaguely remember bugs that I had to work around in MASM 1 or MASM 2, back
when I was trying to write a large 8086 assembler program.  But that was
so long ago I don't remember what they were.

Just because you haven't found bugs, doesn't mean that bugs aren't there.
"conservative" code is a good idea as it will help you avoid bugs...

BillW