Answer:
The strengths of the poster is that it is visually appealing, can convey powerful images and concepts, and can therefore leave an important memory or mark on the people who look at it. It can serve propaganda purposes very efficiently, specially in the case of China's Mao, since at the time many Chinese people did not know how to read.
However, the limitations are precisely that posters are simple, with minimal or no written words, and for this reason, they are not likely to convey an accurate picture of what China's Mao was like. This is precisely why they serve better as propaganda tools than as objective information sources.