I agree with Damascus and his techniques for cleaning a crack, getting adhesive into it, and reinforcing it. Compressed air is also good for getting glue deep into a crack, but can be messy if not done carefully. In clean sound wood, a good epoxy or good wood glue with a clean and properly prepared joint should end up as strong or even stronger than the original wood. This means that a repair in oil saturated wood will not be good, and is likely to fail at some point. Mechanical reinforcements such as pins, staples, and dowels are added to provide extra insurance where conditions are less than perfect for a good strong glue joint. Proper clamping is essential too. Debris in the joint will weaken the repair, and clamping too tight will force out too much adhesive, and starve the joint.
I might be somewhat disappointed to remove the trigger guard on a gun to find that a piece of Baltic birch plywood was epoxied into a milled recess during some past repair. But such repairs can save a broken stock from needing replacement, and are certainly much better than visible pins, dowels, or even the external metal plates, wood screws, and stove bolt repairs that are often seen. In lower grade guns, the cost of replacing a stock may often exceed the value of the gun. Every repair involves a judgement call to determine the best path. There are no One Size Fits All solutions.
I like a good epoxy for repairs that do not show on the surface. That means a high quality clear product like West Systems or Accraglas. The gray and white epoxies containing fillers have no place in repairing fine guns, although we see them used far too often. Epoxy is great because of its' strong gap filling properties when joint fit is less than perfect. Polyurethane glues are also great for gap filling because of their foaming, but they are very messy to use, and more prone to interfering with staining and finishing after the repair is done. But I still prefer using Titebond II wood glue over anything else where the repair joint will be visible on the outer surfaces. In actual side by side tests that I did several years ago to see which adhesive would give me the least visible repair joint in walnut, the Titebond II was the best when compared with a couple different epoxies, polyurethane (Gorilla) glue. etc. My own tests were inspired by the wonderful work done to save broken stocks by guys like David Trevallion and Dennis Earl Smith (The Stock Doctor). I wanted to learn how they were able to repair shattered stocks in a manner that was often nearly undetectable. I later broke my test pieces in a vise, and found that my Titebond II glue joints were stronger than the surrounding wood. I was applying shear forces by striking the edge of my walnut blocks with a hammer in an attempt to break the glue joint. Cyanoacrylate (Crazy) glues are very strong in tension, i.e. resisting being pulled apart, but prone to failure under shear forces. Shear forces are encountered in gunstocks quite often, from things like recoil, a loose action hammering the front of the stock, or dropping a gun on the butt and breaking a piece of the toe.