The problem is the gain and output stage are wide open, so the gain extends to much higher frequencies, where the phase shift wraps around. So you can hardly apply any feedback without it oscillating.
I think the usual approach is to shunt the output into submission with a Big Fuckin' Cap, but this doesn't address the fundamental problem, which is that the circuit itself is unstable and prone to oscillation. Indeed, this tends to result in low level oscillation (~mV), as its screams are muffled into the large capacitor, but not silenced.
You also don't have a driving output, only open collector. So you're limited to class A resistor loaded output.
Used in a proper PWM circuit, it's not bad, but now you're intentionally harnessing its screams, which means the possibility of high efficiency (adding a suitable output stage), and the requirement for heavy filtering.
Tim