The problems aren't with the action economy, or made-up ways to auto-kill foes; it's with the potential to abuse the "game engine" to use videogame language.
That you can use two rods to climb any distance is hopefully obvious, granting you a slow but sure way of reaching any height and pass over any obstacle. Barring a doorway becomes easy, when the door or gate won't budge no matter what.
That having rods encourage you to think about the world in ways it really can't handle is perhaps less so. Rods enable you to put great weights above where a monster is expected to appear. Then the DM must wrestle with unwelcome mixes between game data (such as hp damage) and common sense expectations (such as the expectation that no matter the monster, it will be crushed by a large boulder falling on top of it). Then you can use a rod to utterly wreck something large travelling towards it (like how space dust wrecks space ships) - abusing how D&D isn't set up to handle real-world physics such as potential, mass and acceleration. The "get swallowed, then leave an activated rod inside the monster" strategy is just one such manifestation of the unwelcome invitation to mix gameplay with engine physics.
Then comes the block and tackle engineering, where your player could start reasoning you could set up counterbalances or other contraptions to project immense amounts of force. And the worst part is, he would be right. Being able to have two fixed points in space that will never yield really opens the pandora's box of abuse.
I'm getting a headache just by thinking about it
