After watching these two videos it's unclear to me what is hard about building such engines. The numerical model seemed pretty pedestrian compared to anything else in fluid dynamics. If I understood correctly the detonation is basically stable depending on the right reactant intake. So you wouldn't need submillisecond active control or anything like that.
Is the heat the biggest problem? Do RDE get that much hotter than other engines or produce more vibrations?
I haven't ran the numbers or anything but I would assume supplying the high PSI oxygen for a long burn duration would be a considerable engineering challenge. You would need a system to jump start with bottled oxygen and then a heat exchanger in the combustion chamber to gasify LOX once stable detonation is reached. It is likely New Space companies have run the numbers on engine weight versus a contemporary staged combustion engine and found it to be little to no efficiency gain.
They should come out with a sweatshirt with the name printed in reverse like on an ambulance to not look so cheap production quality.