Just Intonation requires that all notes in a scale have the flexibility to
vary in pitch according to the needs of the harmony at any given moment.
In other words, it's a musical system that calls for harmonically pure intervals at all times.
For that reason alone it's difficult for most modern instruments to play in Just Intonation,
especially instruments that are pre-tuned or fretted (Piano, Harp, Guitar, Bass, Flute, Trumpet).
It is possible for unfretted instruments like Strings (Violin, Viola, Cello)
& the Human Voice to perform in Just Intonation because
they can slide into pitch but it is still difficult (no matter the instrument)
and takes a very refined ear to confidently recognize & execute the correct pitch.
What are Pure Intervals?
*It would also be useful to think of Just Intonation as a 'divisive system‘
... time cuts space into different parts.
This is how we get (& why we use) calendars to coordinate time.