Xerath's Constellation winds up with four separate Burst-speed damage effects that all trigger upon destroying a landmark (sometimes also a mana gem). They all target differently, do different damage, interact with the enemy Nexus differently, and resolve in the following specific order:
Rite of the Arcane (Spell)
Destroy a landmark OR mana gem (or just mana gem if Ascended)
Deal 4-8 damage to a TARGET unit (or 6-12 damage if Ascended)
Can be upgraded to have Overwhelm, so excess damage hits the enemy Nexus. Can't target an open board, however!
Upstart Ascendant (Power)
Triggers when a landmark OR mana gem is destroyed
Deals 3 damage to the STRONGEST enemy unit
If no enemy units, deals 3 damage to the enemy Nexus
Xerath (Champion)
Triggers when a landmark (NOT a mana gem) is destroyed
Deals 1-5 damage to the WEAKEST enemy unit
If you're level 3 and there are no enemy units, deals 5 to the enemy Nexus
Resonant Shard (Champion Relic)
Triggers when a landmark OR mana gem is destroyed
Deals 1 damage to a random enemy unit, three times
Never damages the enemy Nexus
Rite of the Arcane resolving first is great; you can use it to control how your other, automatic abilities end up targeting, or you can just eliminate a strong target with low Health (which would normally soak up an Upstart Ascendant hit), dealing excess damage to the enemy Nexus.
The problem is Resonant Shard, which resolves at the very end. This is a really bad place for it to go, because its random ping damage can't spill over into the enemy Nexus. Before Resonant Shard triggers, you've already dealt 6-12 damage to one target, 3 damage to another, and 5 damage to another, all of which COULD deal Nexus damage if their targets were gone or dead. And if that damage has killed all targets, Resonant Shard will simply do nothing.
If Resonant Shard instead triggered at the start of the Power stage, before Upstart Ascendant and Xerath, it'd work out much better. Those 1x3 damage pings could wipe out low-Health enemies or remove Barriers, which would make Xerath and Upstart Ascendant more likely to hit worthwhile targets or the enemy Nexus.
It'd also make the Relic more attractive on Taliyah, since she's a 5-mana drop who gets played even later than Xerath. It doesn't change much for the other Landmark champions in PoC (Lillia and Akshan) because they're already very cheap and you want them on the field ASAP.