I've had 100% germination with MJ in coco, and nearly 100% with all my garden crops.
Rockwool can hold a lot of water, and if you're not accustomed to using it, it is easy to leave it too wet.
Coco holds quite a bit of water as well, but it drains any excess water quite quickly, pulling a lot of air into the media.
Fill any container with drain holes with 100% coir. Keep at ambient temperature and humidity, water once daily, and you will have incredibly consistent germination results. You might be able to squeeze an extra day by soaking in water first, paper towel method, etc. But all those methods carry possibilities for human error. Rockwool also has a vector for human error if you don't pH it right. Coco doesn't need to be adjusted. Just water daily.
Since I've started germinating and cloning in coco, I don't use anything else.