1 - Grail units, I'm excited to put my NY Eir and Naesala in + spd.
2 - Seasonal Units, are generally more difficult to catch merges and achieve the desired trait, NY Lethe and Spring Fir are cases that I think would be worth using the item.
3 - Units in which a certain combination of trait saves me in long-term merges, Igrene with a trait + atk -spd gains attack point before speed, allowing me to take an additional atk with 6 merges, any other combination would require her to get +7.