Your opponents wont be happy about you destroying their permanents each turn, so theyll likely gun for Reaper King or you before it even hits the battlefield
If you throw a Helm of Awakening in there, you also get infinite mana
The most aggressive versions of U/G Merfolk probably wouldnt want to play Seafloor Oracle, but a token version that is looking to play a longer game with Deeproot Waters and Jungleborn Pioneer could be looking toward this card for card advantage
Yeah, when you have a jackrabbit start against a creatureless deck and you can use this to get your guys back, its pretty good, but against a good chunk of the format, it either does nothing (no enters the battlefield ability) or helps your opponent out as well by giving them back a ton of good utility creatures
From Battle for Zendikar to Ixalan, every block has had at least some nod to white/black draining life (Dominaria has repeatable life loss, but no W/B drain card)
and if they receive enough damage, they die like normal creatures do