As a homebirth midwife and lactation consultant I am seeing more and more of this as well. I also have issue with the hormones potentially adversely effecting supply and there is no data (a study has just begun in Seattle but I am not sure what the study design is and if we will get reliable data from it). I tell all my clients that are considering it that I have seen it seem to help with PPD but because of my concerns about the hormones, I recommend they discontinue taking it if they have any supply issues. I have had women then save it and take it when weaning to help with that hormonal swing, with reported success. One big difference in the animal world is that they eat their entire placenta very soon after birth, they don't dehydrate it and take it over the next 2-6 weeks. 
Take care
Ami Burnham LM RN IBCLC 

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