Lets put it in clear way.
MTF test is the small tip of the iceberg , there is thousand times more problem under it.
MTF test being done with the help of Optical Transfer Function. Think you have a Nakamichi amp and sending its input cable different sinus waves. Every amp make these different sinus waves at a different height at the end.
Optical transfer function OTF describes this phenomenon with the help of two variables.
First MTF , You all saw this graph and phase variable.
You can put MTF with single graph but you can not put phase variable less than 60 graphs. These are aberrations or the bump and the valleys of the distortion on the image.
If you are interested , you will start to not look to mtf graph when buying the lens but the companies photographers portfolio spread to the past and you will start to see the real thing.
Every company have a tradition to put these 60 errors in a combination to create their visual brand.
You can make two same MTF graph lens and they can be extremelly different .
Thats why if you want same character photographs like Ansel Adams , you have to own his lens. Otherwise you end up at another place.