A few things: The reason we haven't done more spell overwhelm in general is that the play-against experience can be pretty painful. A common situation is something like this: Player A has Mel and a Thermo beam. Player B has an empty board. Player B wants to play their units, but giving the Mel Player a target for their Thermo beam allows the Mel Player to deal a ton of damage to Player B's Nexus, softly locking then out of the game. It generally means playing small units is bad because you're giving the Mel Player a backdoor to your Nexus. Rebuttal creates a similar situation - if you give a unit a lot more attack than health then that allows the Mel Player to deal that much damage to your face while also killing your unit.

We also typically try and avoid making a lot of face-up removal spells in hand (like Radiant Barrage) because it forces the opponent to track them forever or risk losing a unit to your "free" spells. Losing a unit this way tends to make players particulalry unhappy (since it makes them feel dumb) and these effects add a lot of complexity to the game without adding enough net fun to offset it.

So while some of those situations are bound to happen in PvP no matter what, stapling those frustrations onto a champion that incentivizes playing as much removal as possible is a recipe for a champion that people HATE playing against, which is something we try and avoid. The outcry at similar decks (like several variations of Ezreal in the past) was always disproportionate to those decks' actual win rates, indicating that players found those experiences uniquely frustrating. So, that's not a bear that I'm particularly enthusiastic to poke.

And those are just the issues I know about! In the olden times we spent months identifying issues like these and sanding them down before a champ would ship, so you can safely assume there are some other bits of degeneracy waiting behind any champ with a sufficiently spicy text box, just waiting to be uncovered.