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).