I'm still waiting for anybody, anywhere, to explain to me how an MTF and someone with CAIS differ as far as the effects of HRT outside of the fact that the MTF was exposed to androgens for a period of time first.
We put all this stock in the idea that our bodies "weren't meant" to have whatever hormones we administer, but that's really just a perception. Unless you believe in god, there's not much "meant", there just "is". And what "is" is that both males and females are humans and respond in much the same way to exposure to whichever hormones are administered, depending of course on the time they are administered.
Now, yes, synthetic estrogenic chemicals like ethyilestradiol or the various unusual estrogens in premarin, certain dosages, and frequency of administration could all have effects in your long term health prospects. But taking into account that I doubt any of us are doctors, what reason would there be that makes XY's so different to XX's, or vice versa, when in the presence of the same hormonal influences?
And anyway, speaking only for myself, and frankly if you interpret any of this as a "more trans than thou" comment, I'm sorry but that's in your head not my intention. I for one DO NOT CARE if it increases my risks of various things. I DO NOT want to live as a male, not now, not ever, so if only for me, whatever risks I'm taking, whatever I might end up having happen as a result of HRT, is still better than the alternative. And I actually faced a situation not that unlike this when I was about to start. I have multiple sclerosis and hormonal influences are absolutely linked to differentiated disease activity in MS, and studies have shown that "males" with low testosterone fare worse and have more CNS damage. But I knew at the time that even if this causes a severe reaction with MS, anything is better than living as a male to me at this point.
And really, if we step back into reality for a moment, there's NO evidence and as far as I know no reason to believe that bioidentical HRT at correct dosages poses THAT much of a risk. The birth control pill absolutely does, but it's ethyl estradiol administered to people who already have a normal hormonal cycle.
So yeah, that's my thoughts on this.