Had to reverse engineer a real mode PCI option ROM once... that was extremely unpleasant [1]. And then of course there's "Unreal Mode".

Moreover Intel is just this week actually finally proposing removing real mode. [2] I'm a bit worried for what this means for emulation of old 16-bit Windows and DOS software under Wine (one of the great ironies that Wine can still run Win16 programs on an x64 host OS when Windows can't) - though I suspect the performance requirements of such software is so low by modern standards that emulating such programs wouldn't pose any challenge.

[1] https://www.devever.net/~hl/ortega [2] https://www.phoronix.com/news/Intel-X86-S-64-bit-Only