Why do some recipes claim to "never fail"?
Baking is a complicated process and requires precision; you start out with complex molecules, and transform them into something has to have many qualities precisely right.

The common quality of all baked goods is the foam structure: each baked good is best when its foam is right. Example: for a cake, the "right" foam has lots of tiny, regular bubbles, and the bubble walls support the foam, but are soft, not elastic.  The ideal foam for a bread or pie crust is different. 

There are other criteria for the cake layer to be good: 


Evenly baked
Regular shape and surface, no cracks or doming
Neither overcooked nor undercooked, starch gelatinized but not retrograded
Moisture content correct, shouldn't be dry or melt in the mouth like a brownie
Color
Balanced flavor: not too sweet, eggy, salty, or bland


This list is just for the layer itself, frosting and combining layers makes it even more complicated!

With so many qualities to balance, you need a precise process to get reliable results; however home bakers don't want to monitor precisely.  Most lack probe thermometers, and Americans often use volume measures for dry ingredients, rather than the more accurate weights.  Finally, most home bakers don't understand the theory of baking. 

Great bakers draw on both theory and practice when they bake, and their recipes rarely fail. Theory teaches them the significance of each step, and the hidden assumptions made by the recipe. Practice teaches to judge what is happening before his eyes, and how to recognize when something has gone wrong.  Theory and practice together let great bakers identify when something has gone wrong and rescue the project. 

A novice will have neither practice nor theory aid them. He is even likely to misunderstand steps.  For example, he may stir vigorously when the recipe says "fold".  He may also omit key steps when pressed for time, without realizing their significance.  For example, using fridge-cold eggs when they should be room temperature. 

Of course, everybody has to start somewhere, and not all baking recipes are created equal. Some have more room for error than others. For example, making a genoise for the first time is very hard.  The egg yolks start foaming at around 50°C, but can't build a good enough foam at below 65°C. They curdle irreversibly at 85°C. So a genoise has to be created in a water bath at a temperature around 70 to 75°C, and when the yolks are foamed, the flour (which has to be low-gluten) is added and mixed well enough to not fall to the bottom in lumps, but not well enough to remove the air from the foam. After that, the layer is baked, and it glues itself terribly to the pan, unless the baker had the foresight to use parchment combined with proper flouring technique, and then used proper technique to remove the layer when it is still warm enough to separate but not so hot that it gets irreversibly squished. There are so many things to get wrong with a genoise that a novice will run into them without ever realizing what happened. 

Recipes that "never fail" are the opposite; they are robust, and don't rely on precision. Simple muffins are one example: The chemical leavening of the baking powder works well in many conditions, unlike the fickle yolk foam leavening in the genoise. Muffins even work if the cook just beats everything together in the bowl. You don't have to worry about overmixing, creaming, or specific temperatures. The fat content keeps muffins moist even when measurements aren't exact, and masks mistakes by enhancing flavor.  The cook actually seeks out doming, rather than avoiding it!  Finally, the crumb doesn't have to be soft, regular and airy the way cake crumb should be.  Muffins can tolerate a lot of mistakes!

To summarize: "never fail" recipes are recipes where:


There are few steps
Each step is simple to perform
Even if the baker makes an error, the end result is still likely to come out tasting good. 


Of course, this ignores the catches!  Publishers can claim a recipe will "never fail" when they are unreliable for a novice.  Sometimes this happens accidentally, when a bad baker repeats a recipe enough that it becomes easy, but others have difficulty with it.  Finally, not everyone has the same criteria for success. I once found a recipe for a wedding cake on a popular site; it used box mix, smothered it in pudding from a sachet, and decorated with gummi bears.  Virtually all commenters insisted that it has turned out "great" and described how the wedding guests complimented them.  I can't imagine that it was really "great", but suspect that people who know what a great cake tastes like didn't bother to make the cake or leave comments.  There is no guarantee that a recipe labelled "always works" lives up to its name, or is actually good.  But, if you're a new baker, it's a good idea to start with such a recipe until you have gathered some experience.