Radix
From what I understood it is still a Hybrid motor so your point, which refers to dangers of a solid fuel rocket is invalid.
From what I understand, the solid-gasseous rubber/polymer engine was suffering from surges or stuttering, especially at start up or shut down.
One possible reason (among many) for such surges, is cracking of the rubber/polymer fuel. If there are cracks and pieces of rubber/polymer break off, the burn will not be smooth and consistent and surging may well take place.
Hopefully, the reasons for this engine malfunction can be discovered, replicated, and cured. A change at this point in time to a liquid fuel engine, would probably mark the end of this project.