. . . that they are totally predictable, and a lot more linear. If you found a +3 sword somewhere in the first game you played, you'll find it there again in all the rest. You meet the same monsters in the same places every game, and you find the same treasures. You also have to tackle the same problems in the same order. They are nothing like as replayable as the MM games.
Wonderful games to play twice – once to learn the system, and once to play it to maximum advantage – but after that, the replay enjoyment you get drops like a stone. I did get some extra enjoyment out of them by playing so that my party could learn all the high-level spells available in EOB3 rather than just a cast-once by reading them off the scrolls, but it was a bit of a chore repeatedly killing loads of demons so that I could do so.