With billet you get a more consistent product. Forging and casting can lead to problems with porosity (air pockets), incomplete molds, errors in the mold, etc.
The big upside to billet is the more consistent internal structure of the metal, and cheaper start up as mentioned.
All the problems you mention above are purely those of cast products. For a forging, you could mention warping during the aging process (anyone reading this who doesn't understand the link between the word "aging" and aluminum forming/manufacturing should go outside and play catch or something) due to uneven internal stresses. That's why a stress relieving heat treatment is specified.
How exactly do you think billet is made? It's cast and forged.... the forging process being "extrusion", in most aluminum.
Billet machined parts are not more consistent or better than forged and machined parts. Period.
Indeed, billet machined parts have crystal structure orientations that match the initial forming process direction for the billet. Forged parts have crystal orientations that match the geometry of the finished parts, thereby reducing the availability of slip planes for plastic deformation.
Casting has an absurdly low cost of entry, too. Send a CAD model to an investment caster, and wait for the truck to show up. Minimal finish machining, and you're done.
In terms of tooling cost, from highest to lowest, assuming that the casting will be jobbed:
Forging (10)
Billet machining (4)
Investment Casting (0)
In terms of material waste, which in high volume manufacturing becomes a greater cost than tooling, from most waste to least waste:
Billet machining (10)
Forging (3)
Investment casting (1)
In terms of mechanical performance of the final part, best to worst:
Forging (10)
Billet machining (6-8)
Casting (5-6)
So, rifles... specifically military rifles, are high volume items for which maximum performance is required, generally irrespective of cost. This solidly suggests forging. In fact, billet machining has all but vanished from serious (read: non-hobbyist) firearms manufacturing. Investment casting is MUCH more popular. It provides a quite high quality finished product with minimal material waste and overall high quality.
Billet machined parts look like RaceFace brake levers, so people think they are *awesome*. Style over substance. It's especially cool when they are anodized purple.
Investment casting can also build parts which are actually impossible using any other process. It's CRAZY what some of the parts come out looking like.
At SHOT last winter, I met the dudes from Montreal who investment cast for basically everyone in North America. They make those silly alloy Glock frames, f'rinstance. (they are not the designers of said silly glock frames, just the jobbers.)
Why do I write these damned dissertations? Is my life so boring?