The problem with trying to breed bettas is that not every male and female you put together will be a match. Although, the male is taking care of the nest, if there's no match woth the chosen female.... nothing will happen. Maybe some shredded fins as a result.
I myself have bred bettas for many years. About three years ago, I've stopped keeping and breeding them in order to create more space for my livebearers. I've sold everything to my neighbours.
In all those years that I've bred bettas, I hardly had losses. In general raising wasn't those fry wasn't that hard. Only in the beginning I had loses.
But yes, be aware that you need sufficient space in order to split those juvenile males from eachother. Btw, sometimes also females can be aggressive towards other males or even females. It really depends on the individual character of those fish.