I'm going to break down a few parts
his tremors
Rek'Sai is QUEEN, last I checked queens are female.
the fact that you have to hit them first means it cant be used to initiate
It's not like her burrowed E lets you go in easily already, not to mention the ult is an EXECUTE spell because it does 20/25/30% of their MISSING HEALTH as bonus damage.
If you want to initiate with an execute ultimate you're playing the game wrong anyways.
the wind up means it isnt useful in a fight because it removes all your damage for over a second
Her roar is 0.35 seconds long, she goes untargetable so she can't be attacked while hunting, then 0.75 seconds later she jumps out of the ground to finish off anything she is going for. So by that time whatever you're chasing is already going to die.
It's a targeted long range gap closer that you can use with your Prey Seeker to set off and as a flash chasing spell if needed. All of this while she is in unstoppable state.
it ends wherever they were when you cast it not where they are at the end.
While she is underground it tracks them, having her launch out of the ground directly toward where they currently are when the 2nd end of her tunnel pops out of the ground, not on cast.
Before you shit post about a champion, learn what they can actually do instead of just guessing.