Taken directly from the website:
It is expected that the presence of the embryos in the maternal environment will lead to the mother being better prepared for, and more receptive to, embryo implantation. The embryos benefit at the start of their life, during the fertilisation and early development, from optimal conditions within the maternal environment. This is expected to optimize the production of highly competent embryos.
So, the benefit for the embryo is still only "expected" at this stage, not proven.
And even then, the potential benefits seem to be expected because of the mother being physiologically better prepared for implantation. So, transferring to a different uterus is counter intuitive.
The benifits to the embryo are not yet even proven, but slready the procedure has been hijacked for parental benefit.
And if the benefits are not established, I don't see how they can be certain there are no risks.