MIDI clock on the Atari ST was rock solid. It being "inherently unstable" is one of those accidental things, like Windows 9x users assuming "computers just crash all the time".
No matter how solid the source of Midi clock, the receiver has to interpolate it’s own clock from multiple midi clock messages. Those messages arrive after passing through a network with arbitrary reliability, latency, and jitter.
The instability is at the protocol level. That is why Midi timecode exists.
The instability is at the protocol level. That is why Midi timecode exists.