Yes, just due to the complexity. Much of the low level stuff would have to be reverse engineered for the later machines, since that is what software survives, but it is not documented as well as the earlier stuff. I also suspect the only language that survived is COBOL, though there was FORTRAN and BASIC (a C was written as an experiment in Santa Barbara and LISP at the U of Utah, but there is no sign that they survived). Burroughs used the concept of "S-Machines" (interpreted instruction sets in microcode) on several of their low and mid range systems. Same idea a P-code. Docs exist for the instruction sets for some of their high level languages (there was no assembly lang for the system). The cold-load on all but the very last of the systems were from cassette. I have them, but need to read their contents.