I agree with you that unbiased information about lecithin is very difficult to find. There isn't widespread doubt that it can be beneficial but because the trials have been poorly carried out they don't carried sufficient medical weight and therefore aren't published.
The most common side effect is allergy and intolerance to lecithin:
1. Because another dietary factor is altering the expected results. Eg other supplements, omissions or extras to you pre-lecithin time.
2. You may be intolerance to the particular type/brand of lecithin as it can be derived from different sources, and also contain trace chemicals/contaminants from its processing.