Until the temperature changes.
Back in 1975 my boss got interested in cheap oscillators, and I looked at a bunch of RC circuits - the 555 got discarded very early. An emitter-coupled multivibrator was a lot better, but even that wasn't good enough - probably due to the temperature sensitivity of the Early effect.
Bob Widlar's LM322 (of blessed memory) got rid of the worst problems of the 555 by using an internal regulator to stabilise the voltage across the oscillator circuit, but it still wasn't as good as the emitter-coupled multivibrator (which probably came from Peter Baxandall).
------------- Bill Sloman, Nijmegen