Stevonnie wasn't necessarily a bad fusion, it was more they were pushed into a situation too quickly and they weren't ready for it. Sorta like when a couple is good for each other but they rush certain milestones like moving in together or getting married.

Good fusions requires a lot of synergy (garnet is rage/passion mixed with harmony/lack of initiative). Learning how to get that synergy to work can take time as you get to know your partner. We see a hint of progress since Steven got Connie to dance in the first place (but they were no where near ready for the spot light)

The way I see it, Connie is focused but fragile (she snaps way too easily) while steven is flexible but cannot stay on the point at times.

Basically, they are like a good pattern welded sword. Connie is the hard, inflexible metal on the edge that can keep things sharp, while Steven is the supporting softer metal that can absorb blows.