Adam, I know what you mean.

Indeed, the flavor is difficult to describe and the fruit has little pulp, but I like it too.
Ubanaxica don´t have fluffis over the skin, is a tad bigger and has a bit more of pulp, with the flavor being almost exactly the same as cabeludinha.
In my house I have a small ubanaxica with 50 cm tall. As a side note, I´m living in São Paulo, where a park named Butantã (famous for producing snake antivenoms), has a few trees of cabeludinha, ubanaxica and also 03 cambucás, and 10 jaboticabas. In july, I was there and collected a lot of ubanaxicas. A lot of peoples here don´t know the fruit, so, I had the time to collect a lot with my kids.
Luc, in fact, I never tasted the guaquica. All that I know comes from internet, but what I read, looks like exactly like you described.
Marcos, I never tasted the guabiju, but now you let me curious about the fruit.