It is potentially devestating, but the cost is high and the counters are there.
Sacrificing a 300+ population will cost the advent player something like 9-10k credits over the next 40 minutes(and btw 40 minutes is how long it took to charge that full strength blast).
Moreover there are counters, a few things to keep in mind:
1) First off Wail doesn't damage units in the gravity well of the planet sacrificing population-only the wells adjacent to it. So unless the enemy has 2 adjacent high pop worlds, if you can manage to get units to the high pop world(perhaps by approaching from a direction the advent player doesn't have vision if the planet is on a border),you've already avoided the biggest danger(as he will only be able to detonate the advacent smaller population worlds.
2). If you don't have a method of achieving #1, don't commit your entire fleet to the attack- heck if the enemy doesn't have a fleet present on the high pop world, sending some siege frigates with a light escort to start killing the populace. You can also try to "force the advent player's hand" by sending a smaller fleet just sfficient to force the player to blow Wail- it takes a very long time to regenerate population...once he wails once any reserve fleet can attack without worrying about another wail from that planet.
3). If you are TEC novalith cannons hard counter Wail
4). If you are vasari you can use a kostura cannon to jump directly to the high population planet without moving through any of the adjacent gravity wells wail threatens
5). Wail can additionally be delayed by sending something capable of disabling structures to the high pop world's gravity well- Subverters for the Vasari, Akkans for the TEC, and Radiance for the advent can all fill this role. Bring some bombers along as well and you may be able to kill the tempel of communion before wail ever goes off. Just delaying Wail can work wonders though- if there's a key battle going on in the adjacent well(the most likely time for a wail), every second you buy gives your fleet in the adjacent well more time to build up shield mitigation
6). Make sure to scout the Advent rebel's planets. If you know the damage potential of Wail, it's not hard to determine which of your frigates are in danger of getting instantly blown up. ANything not in that danger zone may be used to try to force the advent player's hand as discussed in #2
7). Use titans: they are an easy way to give even a small fleet the ability to force the advent player's hand. Also they easily survive even the largest wail.
Don't get me wrong, It's certainly strong, but fairly situational. In all honesty once people get used to it I suspect the main use of Wail will be that it forces the opposition to play in a certain way(witholding the full force of their fleet) rather then because the actual damage will wipe out fleets often.