Unchecked Power

Q: What about other units that die at the same time as Wraith of Echoes, such as to a mass damage spell like Unchecked Power?

No. Units that die also don't "see" anything else that dies at the same time they do.

383.2.c.2. A Game Object will not be able to successfully evaluate its Trigger Condition, however, if it leaves the zone that its Trigger is active from at the same time that its Trigger is satisfied. Example: Viktor, Leader says "When another non-Recruit unit you control dies, play a 1 [M] Recruit unit token into your base." This ability triggers if Viktor is on the board immediately after another non-Recruit unit you control dies. It does not trigger if Viktor and another non-Recruit unit you control die during the same game action (for instance, if they are both killed in the same Cleanup due to the damage dealt by Unchecked Power).