The Phase Missile is one of the main weapons employed by the Vasari. It is the only weapon that is capable of bypassing a ship's shields. Each missile takes the shape of a small, bluish or purple streak, or, in case with strike craft, yellow or purple balls. The probability of each phase missile ignoring the victim's shields and the missiles' damage can both be improved by researching relevant technologies in the Warfare Tree. In addition to bypassing shield health, phase missiles also bypass shield mitigation. The Shield mitigation mechanic also applies to the hull of ships for damage calculation purposes. What makes Phase missiles strong is that they can bypass a damage reduction mechanic.
This makes the phase missile upgrade tree by far the most effective weapon improvement research in the game - and hence are key for Vasari victory, particularly against Advent opponents which have significantly more health invested in shields than hull or armor. It is not uncommon to see an Advent vessel to be destroyed with some shield points remaining.
Rebellion gifted the Advent with a passive phase missile block while in friendly culture. As the block is very meager Phase Missiles are still by far the most powerful weapon against them and, to a lesser extent, against the TEC.
The additional weapon upgrades of the Vasari Loyalists dont compare as they not only come very late in the tech tree and are expensive but also have to deal with shields and shield mitigation, drastically lowering their real damage output. Vasari Rebels however get further damage-boosting research for phase missiles, making them even more effective.