This is wrong.
1) Atmospheric drag (unless you're in really high orbits)
2) Wind
3) Uneven gravitational field. Our planet is not a perfect sphere. The moon is even less so.
4) Solar pressure (though very minor and kinda evens out in orbit).
Space is empty enough and engines imprecise enough that for orbital stuff you have enough margins to correctly predict and plan a trajectory ahead of time, and just do correction burns later if necessary.
On the Moon the gravitational field is indeed uneven enough to matter, but exactly one orbiter really cares about it (LRO iirc has a specific orbit where the unevenness mostly cancels out), most simply rely on occasional station keeping burns.
But regardless, this was a Boeing spacecraft; it wasn't built by NASA (NASA generally does systems engineering; contractors build things like this).
But many things can go slightly wrong when firing all engines fighting your way up the atmosphere.
It would seem we have an existence proof now saying that the above statement is incorrect.
The wording “it would seem” should have tipped you off that no assumptions are being made, but I guess it didn’t work.
And even if we were to make a comparison as you suggest and derive some ratio, that’s not super useful without comparing it to the ratio for some alternative approach that does not use timers.
This is like saying that because the airplane crashed because the wing was incorrectly designed and popped off, everyone should fly biplanes...
We do that. Not with the wings, but with the engines. When jetliner engines were less reliable, we required 3 or more engines on flights that included long sections without passing near an airport (mostly transoceanic flights). Now that some engines are more reliable, we have ETOPS certifications for jetliners using two high-reliablility engines. And yes, there still are two, because no matter how reliable the engines may be, one _may_ fail.
What is backing up that CST-100 timer?
Also, if I have understood things correctly, the lunar lander used a timer based approach to keep the rotation of the vehicle aligned with the surface of the moon as it orbited.
Since the orbital dynamics are very well understood and there really isn't anything that can affect the motion of the spacecraft other than engine burns, there doesn't seem to be any good reason to make it more complicated.