There are many issues regarding reliability here. One issue is that Megasthenes is a fragmentary source. It is extremely tough to determine the reliability of any fragmentary ancient historian/source. A great example is the famous polemic of Polybius against Timeaus. Checkout a scholar like A.B. Bosworth, he did a lot of work on the ancient source material we have for Alexander and the Diadochi. He was prolific on the subject and he might have somewhere made some comments about your query.