Blocking Ultimates
According to Furious Paul's D.va In-depth Strategy Guide, Defense Matrix will block most ultimates in the game, including
- Roadhog's Whole Hog
- Soldier 76's Tactical Visor
- McCree's Deadeye
- Reaper's Death Blossom
- Pharah's Barrage
- Bastion's Tank Form
Defense Matrix will also block the following:
- Zarya's Graviton Surge (before it detonates)
According to PSA - DVA Shield Completely Nullifies Hanzos Ult if Directly Hit, Defense Matrix will block Hanzo's ultimate both when it's an arrow (before it summons dragons), as well as after the arrow summons the dragons:
The PSA is that unlike Genji's deflection which deflects the initial arrow but doesn't stop Hanzo's dragons appearing, the projectile-blocking shield actually stops the entire ult.
However, I haven't confirmed this myself, so I don't know how reliable that comment is.
Blocking Other Abilities and Attacks
Also according to Paul's D.Va guide, Defense Matrix will not work on
- All melee attacks.
- All channeled weapons, including:
- Mei's Endothermic Blaster (short range freeze gun),
- Winston's Tesla Cannon,
- Symmetra's Photon Projector Beam,
- Zarya's Particle Cannon beam.
- Lucio's Blast of Sound (alternate fire).
- Zenyatta's Orb of Discord.
- Symmetra's Sentry Turrets.
- Roadhog's Chain Hook.
Defense Matrix will also block the following:
- Reinhardt's Fire Strike
- McCree's Flashbang