There would be a problem if the person were planning to enter China on the last possible date for doing so. Then a delay (long enough to mean that the flight would arrive the next day) would mean that the person has no valid visa. However, since this wouldn't be the passenger's fault, I'm not sure what the Chinese immigration officials would do. In the extreme, if it were a delay that arose after the flight had left its departure gate (i.e., a delay while sitting on the tarmac), the passenger would have no good options for deplaning. A similar situation could arise if a flight to China were diverted overnight. However, these very rare situations just seem to say that there's some risk associated with planning to enter China on the last possible day for your visa.