If, say 1/3 of the face is quartzite and the remainder is sandstone the miner would be pulling continually right or left of its intended direction. It would likely shutdown once it’s alignment is altered.
I've done a fair bit of reading about these TBMs tonight, and from what I'm finding, the technology involved is somewhere on the order of NASA's. The power, accuracy, and sophistication of these machines is not to be underestimated.