> Tiny tolerance/alignment differences between the target mount and adapted mount can cause the lens to become decentered or be mounted at a tiny angle to the sensor plane.
It is certainly possible to make an accurate mount adapter; see the Pentax m42 → K mount adapter. The "problem" is people don't want to pay for a good one; the Pentax adapter costs about 2x as much as the adapters everyone seems to buy.
I have had mixed results with the Sigma MC-11 adapter. Some lenses worked OK, and some didn't. I don't have the setup or the resources to do a thorough analysis. I attributed them to the 'slop' in the system - From the focus motor to the mount to the adapter to the AF system and sensor. I was probably unlucky and the +/- tolerances didn't cancel each other out.
It is certainly possible to make an accurate mount adapter; see the Pentax m42 → K mount adapter. The "problem" is people don't want to pay for a good one; the Pentax adapter costs about 2x as much as the adapters everyone seems to buy.