I don’t quite understand the theory behind this. Maybe someone could advice me an article or explain briefly.
Why most of analog oscillators are very sensitive to temperature and tuning them is a real challenge while SID is not?
Oscillators are not inherently unstable, but our hearing means we perceive pitch exponentially. So from one octave to the next means a doubling in frequency. If you want to control pitch on a VCO linearly (commonly 1 volt per octave), then you need an exponential converter for your keyboard or MIDI data. Most of these circuits have a temperature coefficient that is always present.
Digital oscillators just count a number of divisions of the master clock. Quartz resonates at a very specific frequency, and so it’s much easier to count pulses this way. I think the SID uses a lookup table to derive the oscillator frequency.
I suspected it is so 
I also looked through a soviet electronic accordion Estradin schematic. It has 12 tone generators heavily stuffed with logic elements.