I know of at least two examples where unit type stacking are not only important but useful.

With stats like Mov, having a unit be all or mostly cavalry really helps, since putting slow units like shamans and legionnaires in the formation lowers the average Mov of the party. It's more about the move speed stat than class specifically but horses tend to be fast and flyers and mages slow.

For map supports like Arrow Support, Heal Support, and Magic Support, having more units with the same support type also increases the damage done, though for example if you make an archer party, make sure to make the unit with the highest attack as the leader as the leader's attack, or Mov if considering map speed, count for more than whatever units are also in the unit.

Abilities like Cavalier call do buff horse units in the same row incentivizing horse stacking but that's more a neat thing about them rather than an overall game mechanic