I think like most things you have to make assessments for each individual situation. I bred my 17 yr old mare on her foal heat and she caught. I was suprised, she carried and foaled out just fine.
You have to assess your mare after she has foaled to determine whether she would be a good candidate. If she cycles quickly 4-5 days after foaling I would avoid it. Some mares will cycle up to 7 days or longer, and tighten up quick, these types are more likely to succeed. If your mare has a difficult pregnancy and there is any swelling, or damage I would NOT breed on the foal heat. If your mare’s physical appearance is less than ideal I would not breed on the foal heat. I personally wouldn’t breed frozen on foal heat. (but I have had crummy luck with frozen at the best of times)
So if your mare is a fertile mertile, she has a smooth easy delivery with no tearing or damage to cervix, vagina, or vulvar lips, she is healthy then IMO there is just as good a chance for her to conceive.