As has been stated, the IGI contains a mixture of information. Part is direct from records under the name extraction programme. This can be regarded as highly reliable but even here it is advisable to check to source records as mistakes occur occasionally.
Part derives from member submitted information.
Here the difficulty arises. It is simply a record of information submitted for Temple ordinances which happens to be very useful for family history research. It was not created purely for research purposes. The standard set by the LDS Church as to what can be submitted has varied over the years. In early periods almost anything was allowed.