Theoi is useful and incredibly convenient, but it it is not definitive, nor peer-reviewed:
- Listed passages on a given subject are often assumed to be the complete body of material, which may not be the case.
- Only a single translation of a given passage is offered
- As these translations are often old (i.e. in the public domain which is why they may be utilized) and can contain inaccuracies.
Theoi, like Wikipedia, is a great starting place, and serves an online function similar to Graves' The Greek Myths with it's useful index, albeit in a much more limited capacity, containing only a fraction of the subjects covered in Graves, and no peer review.
In other words, Theoi is not an academic source, but it is commonly treated as one: https://mythology.meta.stackexchange.com/a/265/2892