The Rocketdyne Tripropellant rocket had great specific impulse, one of the best. But-- there are many reasons it never caught on: one of the byproducts was FOOF, along with other things like hydrofluoric acid.
If you want to actually get momentum out of a rocket, the reaction products are going to touch the combustion chamber walls and the nozzle. While film cooling can help with minimizing heat transfer from the hot stuff, I doubt it’s enough to keep this stuff from eating your engine from the inside.
"These modern SpaceX kids and their fancy reusables. Back in my day, when we went to space, we used the whole engine!"
That’s what engine-rich exhaust is for.
Hmm, I wonder if anyone has tried to use ablative coatings in a rocket engine.