I don’t think it’s necessarily accurate to say that FSRS “struggles” in those situations or that they “mess with” the algorithm.
The reason to use shorter steps is the clearest. Using longer steps interferes with FSRS taking over and scheduling the card. If everything is working correctly, FSRS will do a better job scheduling a card for 1d+ intervals than you would do by trying to control that with fixed steps. [If everything is not working correctly, it’s better to fix how FSRS is working for you, than to push FSRS out of the way and do it yourself.]
As far as the reason to use fewer steps – this recently came up in another thread, so I’ll quote myself as a jumping-off point –
Why one step though?
I make that recommendation based (1) on FSRS research that found multiple same-day steps have little effect on long-term retention, and (2) on FSRS functionality that considers multiple grades in Learn to set the first interval when the card graduates to Review.
For some users, too many correct grades in Learn can cause a drastic increase in that post-Learn interval. All of the analysis of FSRS points to that effect being correct, based on those users’ review histories (meaning, it’s not a “bug”[1]) – but that doesn’t make it look any more rational when they see it.
A single step, as a starting point, deals with both points. Some users may find they need to add an additional step, shorter or longer, to deal with issues they are having in Learn, but that can’t be known until they get started.
Is it part of FSRS not having a model for near-term memory? Does that make it so FSRS isn’t able to consider those near-term/same-day grades appropriately? I’ve asked those questions and been told that isn’t an issue. ↩︎