I guess I don't see the issue with cast for receivers.
OK, so here's how this works, in very simplified terms.
Forged steel is a solid, hardened chunk of metal that the final receiver is cut out of. A steel forging, for the most part, can be treated as a known good quality. Changes to the good steel are made by cutting away the unnecessary parts.
Cast steel is essentially poured (keeping it more simple than technically correct here) into molds in the shape the part needs to be. It is then heat treated for strength. A possible problem is that voids will form inside the metal based on how pure the metal is, and the processes and quality control used. A casting can be essentially as good as forged, or as crappy as the zipper pulls on luggage that break all the time.
Compare it to reloaded ammo, and you won't be far off. Can reloaded ammo be as good (or better) than Federal Gold Medal Match? Yes. Does it mean that all reloaded ammo is better? No. A person using meticulous processes and double-checking each step will probably make better ammo than Federal's machines. The guy selling it in baggies at the flea market? Probably not. You probably don't want it too close to your face when you pull the trigger. The only way to tell if you can trust the ammo is the reputation of the manufacturer.
The same is true of casting. As far as I am concerned, Ruger has mastered casting to the point that I would not hesitate to trust their work. On a knock-off M1 Carbine that you don't even know the name of the company that cast the receiver? Not so much. Or a company that is relatively new at making the product and you don't have a good history of their work? Maybe.
M1 Carbines don't have a lot of bolt surface area, and if the hammering of the lugs stretches or distorts the receiver it can change the headspace. The quality of the metal determines if and when this will happen. Forged receivers are (for the most part) uniformly made of consistent steel. Cast receivers are made of steel whose strength and consistency are determined by the manufacturer's attention to detail and quality. If you trust the manufacturer, no problem.
With that said, I have an AO carbine I use and like. I keep an eye on headspace, but I am still wary of it. A Ruger Mini? I just keep shooting it.
That kinds went long, and that was with me oversimplifying details to a ridiculous extent. Either way, that's my take on it.