Final answer:
MacConkey agar is a selective medium that inhibits the growth of certain bacteria and promotes the growth of others, making it unsuitable for comparing overall growth.
Step-by-step explanation:
The reason it wouldn't be advisable to compare growth on MacConkey agar is because it is a selective medium that inhibits the growth of certain bacteria and promotes the growth of others. MacConkey agar contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria and favor the growth of gram-negative bacteria, particularly Enterobacteriaceae. This selective property makes it unsuitable for comparing overall growth between different types of bacteria.