Have skim read both threads and actually know a couple of the instamums mentioned personally (and they are both lovely people). A few points spring to mind:
Instaparents are brands in themselves.
People have emotional attachments to brands (Apple, Coke, John Lewis, etc).
People form emotional attachments with brands they perceive to be authentic.
When those brands move away from that authenticity, they lose trust and people feel manipulated. Brands/instaparents ignore this at their peril.
With the exception of MotherPukka (that I know of) none of these instaparents have a cause or a point of view beyond 'having kids is hard and my body is shit but I'm okay with it'.
The 'having kids is hard and my body is shit but I'm okay with it' angle can ring a bit hollow when it's blatantly obvious that these women are beyond averagely wealthy, even before they get paid for endorsements, and they are also above averagely good looking.
There is no acknowledgement of that privilege, which can be grating.
Brands are bought and sold on the size of their audience/consumer base. As the target audience for these brands, the opinions on this thread (when not personal) are totally valid and the instaparents should be analytical in taking on board the feedback rather than emotional and defensive.
If you lay your family life bare on the internet, at some point you are going to lose control of how people receive what you're putting out there.
There are a lot of very strange people online who would say weird shit about your children, stalk your house, etc. Personally that would worry me enough to make me not want to do it.