One limitation with smaller scales is that the small size of the metal contacts means it is easier for dust and dirty track to interfere with the electrical circuit needed to drive the train. Moreover, the tight tolerances also mean friction can more easily interfere with the train moving.
A limitation of miniature models is that the small size of the metal contacts makes them prone to interference from dust and dirt on the tracks, which can disrupt the electrical circuit needed to drive the train. Furthermore, the tight tolerances also increase the likelihood of friction hindering the train's motion.