ARCA Space's webpage for the Stabilo program has a description and an image. Hydrogen peroxide monopropellant? Reaction closer to the crew than the nozzle?
With thrust at the top, it seems that
simplification due to inherent stability is a big advantage no, per @RussellBorogove's comment that's not right. While the word "Stabilo" appears frequently, the "aerodynamic stability system at the top" does not refer to any passive stabilization due to thrust-at-the-top at all. It's something different entirely.
Why was the thrust put at the top, and how was the attitude controlled during the phase of descent that used this engine?
Also, wasn't it tempting to use all that free oxygen to get a boost from combustion? Or was that going in the wrong direction - adding complexity where simplicity was sought?
The problem with steam is that it condenses, shrinking by a factor of roughly 1000 in volume. Was it difficult to ensure that the steam was always above its local boiling point (noncondensing) at each location and pressure, at least until well outside of the nozzle?