I've only experienced Nightmare Mode on something like JB_fuckzz_final (Don't question me on that, I suck at map names.) It was entertaining and people on chat and mic seemed to enjoy it, but like other people said, it would be very hard to make and they would have to edit every map to apply...

Demoknight is bad for guard, but if I had to give my personal opinion on the worst loadout, it would be Soda Popper, Crit-a-Cola and The Boston Basher.