It's a different way of cooking; closer to baking than stove-top cooking. You need to get the quantities right. If you follow some recipes for a bit you get the hang of the ratios required for a lot of things, or just continue to use recipes.
Baking at least you can open the oven to take a quick look, and even use an oven thermometer to see how well done it is. (I've recently discovered that you can use an oven thermometer when baking bread, which has been amazing!)
With a pressure cooker, it takes several minutes to depressurize, and the required seal precludes using a wired oven thermometer. So you just have to kind of guess and hope, which often means things are slightly over-cooked (since that's safer than slightly under-cooking).
