An RC reset doesn't too often have a problem with the length of the reset pulse.

Electronics have minimum requirements but very seldom maximum reset pulse length.

So it is (relatively) easy to get a reset pulse to work in laboratory environments. Most (but obviously not all) power supplies have fast enough ramp-up of the voltages that the ramp-up isn't the big issue.

The really big problem is that since it only uses passive components, it suffers from a "memory effect". It can't instantly arm itself for a new full-length pulse in case there is a tiny glitch on the input power.

It is quite common that the power switch results in bounces on the power, in which case the RC reset "consumes" itself based on the first power on, while the microcontroller needs a proper reset pulse timed from the last of the multiple power-on events.

With a proper reset chip, the power is either good enough to keep the hardware running, or (if there is even a tiny glitch below the rated supply voltage) there will be a full reset pulse counted from after the supply voltage has reached acceptable levels.

With a RC reset, there may be a danger zone from microseconds and up to seconds where the RC solution will not give a proper reset. So people have to learn to disconnect power, count to ten and then turn on power. And hope that there wasn't any power bounces when the power plug was inserted or the power switch pressed.