Fancier power steering systems basically fully disengage at speed because you don’t need them; they’re for moving the wheels when you’re stopped or nearly so.

I guess I got the wording wrong because that's exactly what I meant.

The most dangerous situation I encountered was turning left across traffic from a stop.

Yeah that's exactly what I was thinking about (and thought I wrote, but the downvotes say otherwise).

If your steering assist conks out just as you're trying to turn at slow speed you could end up heading into oncoming traffic, at an angle at least.

It's actually pretty dangerous, good thing you don't drive it any more.

A combination of whatever fiddling the mechanic did and making sure the fluid was topped off seemed to solve the issue. Also, it was a good, if somewhat old, car, and the mechanical steering was quite good. I was able to steer it without much difficulty at pretty much any speed even in no-power-steering mode.

But yes, I agree, unpredictable steering response is dangerous.