I started with a humus based soil, just like most people. I have found out that if you do not repot regurlarly, in the long run, all your cacti will suffer and die (even Optuntia's). Somewhere around 2005 I began to change to a mineral soil. Initialy this was pumice with some sand and clay. After I ran out of sand, I shifted to pumice and clay alone. But with larger pots I was not satisfied with this mixture. So I began to include lava in the mixture, as this wa a bit coarser. My mixture now contains about 1 part lava (4-12 mm) and 2 parts pumice (1-8 mm). For Opuntia's I mix about 5 % loam to the mixture. I don't think this is needed, but it gives my plants an extra boost for growing.
Other people grow on flugsand (which has also a vulcanic origin) or just coarse sand. Outside (at least in The Netherlands) it is different. And where I used to grow polyacanthas and aurea's outside, I have now the greatest problems with even phaecantha's. I know that outside loam/clay is a very bad choice. You want to drain as much as you can. I'm currently experimenting with basalt split and sand.