There is some evidence suggesting that transferring frozen blasts has higher success rates than a fresh transfer. If you want to do genetic testing, which could increase your chance of success, you pretty much have to do a frozen transfer. Testing aside, though, the hypothesis is that there’s something about the stimulated hormonal state following an egg retrieval that creates less than ideal uterine environments for embryo transfers.
https://www.ccrmivf.com/services/ivf-fertilization/frozen-ivf-advantage/
The downside to a frozen transfer is (1) cost, (2) time, (3) the possibility that your embryos may not make it (or fewer of them would make it) to day 5 to freeze, and (4) not all frozen embryos successfully defrost.