CityLab researchers exploited MD5's weakness to answer questions about the system. While not a real problem in practice, it seems clear MD5 was not an ideal choice.
From the article, the researchers generated forbidden and allowed images with a colliding hash to prove WeChat was using MD5. The allowed image was banned in the future as a result.
However, MD5 collision generation has some constraints. It's very hard make an image collide with a particular known hash, but it's feasible (5 hours with a large GPU) to take two images and modify them until their hashes collide. Practically this means exploitation opportunities are rather limited, but a forced collision being possible at all seems non-ideal for an adversarial use case. There's also the risk that future cryptanalysis will further weaken MD5. Seems clear to me WeChat just should have used something like sha256.