What is the "ease hell" problem with SM-2?

Because SM-2's interval depends entirely on the ease factor, repeatedly pressing "Hard/Again" can drive a card's EF down toward its 1.3 floor and trap it at painfully short intervals forever.

Each mediocre grade shaves EF; nothing routinely pushes it back up. A card that goes through a rough patch can end up permanently over-reviewed even after you've mastered it — the algorithm has no memory of the fact that you're now recalling it fine, only the scar tissue of past grades.

More broadly: EF reacts to your grades, not to the actual probability you'll recall the card, so SM-2 tends to over-schedule easy cards and under-schedule hard ones.