Essentially *all* fields overproduce PhDs relative to the number of tenure-track positions out there. Given that faculty can have decades-long careers and the increase in available tenure track positions is slow, anybody producing more than one or two PhD students is probably overproducing. But faculty are rated in part on the number of PhD students they graduate, and in the sciences there's an expectation of very high publication rates to get tenure, which leads to large groups and probably more overproduction than in the humanities.
The issue is really more that much of academia (including a great deal of the sciences) considers students a failure if they don't end up in a tenure track position somewhere, and students buy into it. PhDs in the hard sciences and engineering tend to have low unemployment, but it results from people shifting into industry or government jobs of various sorts, often that pay much better and have more mobility (and not significantly less job security). Tenure is overrated -- tenured faculty tend to have relatively low (and slow) mobility compared to industry, and pay scales in industry tend to be much better. I know quite a few tenured faculty who feel more or less trapped in the institutions where they were tenured - tenure is a big commitment for institutions (and tenured faculty tend to want big startup packages to move) so there tends to not be a lot of moving around except among the top ones who get recruited from place to place. In principle, tenure gives you a lot of flexibility in your research, but it's still limited by what you can convince a review committee to rank highly enough to fund, so money tends to follow name recognition and familiar research.
The separate problem that humanities has is that many, if not most, students pay for their own advanced degrees, where in science and engineering you're paid (not highly, but enough to live) to get your degree. If you're in a technical field, your undergrad loans are getting to look less and less expensive as you get deferments while in grad school and aren't racking up any additional debt. In the humanities, you tend to just be adding more debt on top of the undergrad loans. There are plenty of jobs that humanities PhDs can do just fine that probably pay better than faculty jobs-- what's needed is a cultural shift that says "you don't have to do research on whatever you did your PhD in for the rest of your life, or even research at all. A PhD is a demonstration that you can do unique, intensive research in an area and makes a contribution to the knowledge in an area. It shows that you can read, write, and think independently." It shouldn't be treated as trade school for whatever narrow subject you wrote your thesis on.