I don't think she belongs in the Unstable category. While her personality is unstable, she, as a fusion, was very stable. She was stable to the point that only physical harm could separate her. She had her own unique personality. She wasn't showing signs of Garnet, nor was she showing signs of Amethyst, she was her own fusion.
But if you think I'm wrong, could you possibly elaborate more on how to be an "unstable" fusion? Do you define it as the fusion's integrity or personality?
Today's episode confirms this as well. Sugilite is a stable fusion. They just don't fuse into her because of how unstable her personality is, not in a literal sense but more of a figurative one. One that means that she is prone to going berserk or ballistic.
Sugilite has her own unique personality. She's stable. Malachite seems unstable but is quite stable actually. The only problem with Malachite is that, even though their bodies are fused, their individual personalities aren't, in this case Lapis is repressing Jasper down. In any case, Malachite's more of an incomplete fusion than being an unstable one. Because if their personalities do combine, there is no longer a Lapis, nor a Jasper, there would only be Malachite.