Propyne is a light hydrocarbon which packs quite a lot of energy in a triple carbon bound. It poses no significant health or explosion hazard by itself, and some European studies have found it to be a good potential fuel.
It is an energetic, relatively dense fuel, though it burns rather hot. it also exists in equilibrium with propadylene, though I don't know what kind of impact that would have.
However, designing engines for a new fuel type can cause significant challenges, not all of them obvious. So let's assume that some company decides to follow the steps of Vector, but go for propyne instead of propylene. Leaving aside the production of the fuel, what would be the difficulties in designing such a new rocket?