The PCU was a technical fix, the investigation process worked and they found the problem part in the PCU.

The MAX is more insidious, like the PCU mishaps, it was easy to find the offending system (or lack of redundancy). I believe with MCAS, Boeing misplaced the confidence that the FAA granted after (arguably) abusing it’s limited self-certification authority.

The result of the investigation produced a quick technical fix, although the FAA seems to be treating the mishap as an internal human factors stand down. I don’t expect Boeing will enjoy the ability to self certify for quite a while and the delay appears to be the regulator trying to re-write its procedures on certification. My guess is the reason it’s not going faster is probably the same reason that they allowed Boeing to self certify in the first place - no in-house talent with any real depth.