That is a criticism. For example, Porsche has a very small market share of the general car market, so BCG would suggest that Porsche is a problem child or dog. However, it has a very large share of the high performance car market and that would imply a star or cash cow. Therefore, there can be a radical change depending on what you perceive as the market and your market size. (Porsche is very successful and has been for years, so the proper market would be the high specification/sports car market).
thanks for highlighting the criticism. So looking at the porsche example, so it would be very difficult to classify a product in a market using the bcg matrix.