Depends what creature you're singing at, and what your opponent's shield is.
If your opponent is using Permafrost Shield, you could freeze something of his. If Fire Shield/Thorn Carapace, you could kill something slowly. If Procrastination, you can lock a creature out. If the creature has any poison counters, they will be applied. Throw on your own Thorn Carapace (ooh,
added synergy!), and you can speed up some of the kills it grants. A fun thought - combine this with Gravity Pull and make your opponent's creatures fight each other! Some of these ideas imply trio decks, which makes this card look more interesting in light of the new Pendulums.
Siren does its own damage in addition to being able to "reflect" a creature's damage for 3
a pop. If your opponent has a Ruby Dragon, that's 15 damage for 3 quanta - the efficiency would be equal to a Fire Bolt with 40 quanta stored. Like PU, this can be a disincentive to use Growth excessively. Of course, you wouldn't sing at a
Photon for the sake of the damage...
But yes, this does have some interesting potential to screw your opponent over. Take away the ability and it's still 4 attack for 5 quanta and fairly resilient - it's like an Abomination.
A little quirk you may not have noticed: even friendly creatures can be targeted. This means you can use Antimatter against your opponent, and speed up freezing/delay counters on your own critters.
I did consider lowering the ability cost, but I thought about it for a while and I think it's fine as is. Thanks for the input!