I measured 6 different ESCs (all different types) at 25° with a 16MHz ceramic resonator, and standard deviation was 1200ppm, or 0.12%.
The ceramic resonator is the component in the pic near the right hand lower corner of the MCU chip and connected to MCU pins 7 and 8. They are often referred by RC buffs as “external oscillators”, but the oscillator electronics is in the MCU, just the frequency determining element is an external component.
Quartz crystals (as might be used on FC boards and some ESCs) might run closer to 30ppm.
Both ceramic resonator and crystal are much better tolerance and stability than would be expected of the internal RC clock, especially where it is raised to near 16MHz using the calibrate function, and stability assists in maintaining throttle calibration. Cheaper implementations tend to use the internal RC clock.
There is no specified accuracy of an Atmega8 internal RC clock with max value for the cal word. I have measured a single example at 2% low. No specifications are given for temperature stability at 16MHz, but at 8MHz it is about 600ppm/° and likely worse at 16MHz.
The clock used in most Silabs C8051F33x based ESCs is an internal RC clock quoted at 2% accuracy, and 50ppm/° temperature sensitivity.