I've been reading this thread with interest, having never formed a firm opinion on surrogacy.
With adoption, any negative impacts on the adopted child are likely to be acceptable because the alternative of not being adopted (eg a childhood spent in care) is even worse.
With surrogacy, the situation is quite literally man-made.
I don't know what the psychological effects of being a surrogate child are, and I suspect these effects are variable and unpredictable, but in the event that a surrogate child suffers MH trauma, what is the point of a lifetime of "privilege" and financial security if they're not happy within themselves?
To say that surrogacy is fine because many children are born naturally into rather rubbish circumstances, is an illogical argument.
The fact that many children may be born into rubbish circumstances is terrible but it doesn't then follow that it is okay for other children to be deliberately born into different rubbish circumstances.
Currently, it seems impossible to fairly safeguard everyone's interests (and in particular, the surrogate mother and child) should anything go wrong.
Like many things, a good indicator of whether surrogacy is an equitable arrangement or not is: is it equally available to (and equally taken up by) a good cross-section of people on "both sides" of the undertaking.
ie, are there roughly equal numbers of wealthy/white/western/privileged/etc... would-be parents AND surrogates... or is there an imbalance somewhere
And if there is an imbalance, then one group are probably at a disadvantage...