Hi Lily 🙂
I think it's a good way to have a foal from a mare without having to stop her competition career.
The gestation period for a horse is 11 months, so you’re only getting one foal per year when you breed traditionally.
This process can be done on younger mares with no interruptions to their competition and training schedules.
The process not only keeps mares competing, but it allows stallions to cross continents. Frozen fertilized embryos from working polo ponies in the US are now being shipped to Argentina where they are carried by mares and then trained by some of the best polo trainers in the world. On the flip side, semen can also be frozen and shipped to the US.
I found this article about embryo transfer and surrogate mare: https://thehorse.com/149225/surrogate-mares-impact-on-embryo-transfer-foals-evaluated/
It says that the breed of the surrogate mare has an impact on the foal, for example at birth, pony foals from draft mare surrogates were 57% heavier than pony foals from pony mare surrogates.
I think it's a good thing if it's done carefully. If someone wants to have a foal from a certain breed for example, he won't have to buy a new mare if he already have one.
If the mare is not a 'good mother', or if the foaling might be complicated, it's also a safe solution to have a foal from this mare.