Those go into a little more detail and is cumbersome to read unless you really want to do this and then I recommend reading everything you can get your hands on, so I'll "dumb it down" lol.
Some of the big things that make it difficult are when initial fertilization. If a clam is stressed, it will release sperm, this can trigger all other clams to release their gametes falsely. This can quickly destroy water quality and be a disaster. It doesn't take much to stress a clam either. I had one for awhile that would release every other day, I had to get rid of it. It's easy to get a clam to release sperm, getting them to release eggs can be an issue, luckily I had mature clams that had been successfully bred by someone else before me, and I paid dearly for them. The next issue is that they don't lay eggs. The sperm and eggs must meet in the open water, so this creates a much trickier fertilization process than say clownfish that lay eggs and then fertilize them. Then you need to get them, that's always fun. They will not attach to anything for 7-10 days and they are TINY so this is probably one of the hardest parts, keeping water flow, but not enough to loose the babies. Then comes the issue of feeding them zoozanthellae, trying to find good strains can be a nightmare depending on where you live.
Hope that helps a little. That's just the starting point.
If I ever do get back into it, I'll start again with Maxima clams, they're the only ones I've ever bred successfully and then I'm going to try hybrids (Maximas crossed with Crocea) as you can get some really crazy colors that way. There is a man in my reef club that just started crossing and I am really watching closely to see where things go.