It's not so much that it's hard to figure out, it's that it's hard for a Regular Jane assembly programmer to know how to do well. There's an embarassment of modes with complicated performance characteristics, and a lot of decisions to be made.
So yeah, serious domain experts and modern optimizers can do really well with the architecture, but it's a huge pain to try to "get stuff done" in, and it's from an era where serious Stuff was still gotten done in hand-coded assembly as often as not.
Basically: with a Z80 or 6502 or 8086, if you had a task "X" to do the "best" way was as often as not the obvious way. With the 386, that was suddenly no longer true and you had to have a bookshelf full of Abrash tomes and whatnot. It was a very different feel.
So yeah, serious domain experts and modern optimizers can do really well with the architecture, but it's a huge pain to try to "get stuff done" in, and it's from an era where serious Stuff was still gotten done in hand-coded assembly as often as not.
Basically: with a Z80 or 6502 or 8086, if you had a task "X" to do the "best" way was as often as not the obvious way. With the 386, that was suddenly no longer true and you had to have a bookshelf full of Abrash tomes and whatnot. It was a very different feel.