I can speak at length to the tenure of Kim Foxx, Chicago's former high-profile progressive prosecutor. I know some of the issues rhyme with Boudin's term, but San Franciscans can tell his story better than I can.
So, first, no, I feel like I'm saying the opposite of "they're dumb". I don't think either Foxx or Boudin are dumb. I think they're both interesting people with interesting and valuable views.
When I say "basic competence issues", I'm talking about the kinds of things that would go wrong if, like, you or I took over the CCSAO and started managing all the prosecutions in Cook County. For instance: having huge numbers of line prosecutors resign, in part because you totally fuck up the promotion ladder, in part because you shift staffing priorities away from line prosecution and towards internal policy positions, and in part because you fail to sell your immediate-term vision for how you're going to manage the agency.
The superficial way to look at veteran prosecutors resigning is that they're no longer culture fits, which you can look at as a good thing: Boudin and Foxx were hired to change those cultures. But a more practical and immediate way to look at them is that losing veterans puts the screws on your ability to execute the day-to-day of the agency. These prosecutor offices were incredibly strained before people like Boudin and Foxx got there. Which means: there was already an extent to which prosecution decisions were being made not just on justice, safety, or public policy more broadly, but simply on a triage basis.
When you start losing significant numbers of people, you lose the ability to set your own execution priorities; circumstances are making prosecutorial decisions. Foxx tried to put a brave face on it, but nobody was buying it.
What's more frustrating is that Foxx was doing this at the same time as Illinois was rolling out SAFE-T, which ended cash bail in Illinois. I am wholeheartedly in favor of SAFE-T, and I think by-default cash bail is an idiotic system that unnecessarily amplifies the societal cost of law enforcement. But SAFE-T was ultra-controversial in Chicagoland, and Foxx went through all this stuff while people were freaking out daily about catch-and-release. It didn't help that all of this coincided with a huge regional increase in carjackings, the second most important urban index crime after murder. It further didn't help that she was accused of refusing to prosecute juvenile carjackers, and that when confronted by reporters about that, she didn't have a clear denial.
I hope this reads as I intend it to, which is: not ideological, just an assessment about whether someone is prepared to step in and run the office, most of which is boring and just needs to be done correctly.
(I think you can probably look at Krasner as an example of a prosecutor who has avoided these traps.)