The effect that adding flour or water after initial mixing will vary depending on what you are trying to build. Different techniques leverage different properties of flour, water, and fat and so will behave differently.
The differences come down to fat and protein. I'm sure you know that wheat flour contains proteins (glutenin and gliadin) that, when mixed with water, form stretchy gluten.
In biscuits, quick breads, cakes, etc., the stretchiness of gluten is very undesirable and the techniques focus on minimizing it.
The biscuit method is cutting fat into the flour and then mixing the liquid in very briefly. The lack of agitation minimizes the gluten formation.
The muffin method involves beating sugar and fat and then mixing in the flour and liquid. The sugar makes the fat fluffy which is vital to the texture. The large amount of fat coats the flour and prevents it from mixing with water.
Bread dough is completely the opposite. Gluten makes bread what it is. The more gluten the better. Flour is mixed with water and either kneaded or allowed to sit in the fridge for a long time to maximize gluten. Fat is sometimes used for flavor or to soften the bread but it is mixed in completely.
The implications:
For doughs that rely on minimizing gluten by minimizing mixing, adding more flour or liquid is undesirable.
Adding the flour a little at a time means that it will have a lot of time to be kneaded and the product will be much tougher.
A good example is the second rolling of a batch of biscuits. The first rolling is always lighter and fluffier and when the scraps are rolled out again, the dough forms more gluten and the second set of biscuits are tougher and don't rise as much.
For doughs that distract the gluten with fat adding more is fine within reason.
Mixing will create more gluten but if it is still coated with fat it won't be too bad. It is bad if the ratio of fat to flour is thrown off. I can't tell you how far off you can get and still have a good product.
With bread doughs, flour and water can be added at any time and still be good.
The problem is purely mechanical. It's easy to add water to flour. It's easy to add flour to bread dough. It's hard to add water to bread dough- the water just splashes around and it takes a while to get it integrated.
When kneading bread by hand, it is good to start with too little flour because adding water is so difficult. In a mixer it's not so bad because it's doing the work and the mess is contained.
Remember that flour and water have almost no flavor and adding too much will dilute the flavor of the product. Even in cases where the flour itself provides much of the flavor, as with yeast risen bread, there won't be enough salt. How much you can dilute the recipe with flour depends entirely on the recipe and your own personal preference.
