Most NT files are stripped. This means that trying to disassemble them is a bit annoying because there are no symbols available. Checked builds of NT came with the symbol files (e.g. support/debug/ppc/symbols/exe/ntoskrnl.dbg for ntoskrnl.exe), but tools like Microsoft's dumpbin or OpenWatcom's wdis don't use them.
Now there's https://github.com/andreiw/dbgsplice to add the COFF symbol table back!
Sadly, the OpenWatcom analogue is quite buggy, so it's hard to suggest. It was a capable disassembler around setupldr and veneer.exe, but it gets horribly confused with complicated section layouts.
Of course the DBG files contain quite a bit more info (and we can do a lot more with the aux COFF syms too for annotating code than dumpbin suggests).