Let's see I think casting is OK as long as it is done right with good materials. You may have more chance of finding defects, such as non metallic inclusions and porosity, however. Porosity, as we all know, will affect the strength of the materials, and those non metallics inclusions, if not treated correctly, will act as stress concentrator and act as crack nucleation sites. Both porosity and inclusions will cause aluminum to have shorter fatigue life (please allow me to remind you that Al does not have fatigue endurance in the first place). The grain structure of aluminum casting is also questionable if their molds are to thick or if they have really low thermal conductivity: large columnar grain will form, causing the product to have very poor mechanical properties. I guess the manufacturer can always mix titanium or other metals in the molten almuminum to provide lots of heterogenous nucleation sites for the solid phase to form, thereby, reducing the grain size of the casting. Afterward, the manufacturer can then heat treat the casting and artificially age it, making it stronger.
The reason forging is so good because the blow holes (porosities) woud be weld shut, and some types of inclusions can be made into stringers, in which with proper orientations, can acually act as crack suppressors instead of originator. Furthermore, forging will give you small, recrystallized, equiaxed grain structures, which is really good for the mechanical properties.
Bottm line: metallurgists don't really like CAST ALUMINUM for parts that will be subjected to high load levels, because of the fear that they were not done properly. Mandatory lecture is always about a busted cherry picker with badly cast aluminum lugs. But as far as lower receivers are concerned, I think it should be totally fine, PROVIDING THAT IT WAS DONE PROPERLY. If cast parts are not properly casted, all bets are off! On the other hand, if cast lowers are cheaper, and they're reinforced (say with thicker walls) at the critical parts, then I wouldn't worry about it and buy it. Another thing is that cracks will propagate SO fast in bad casting, it will fail in a brittle manner, i.e., without any warning none whatsoever.
I hope my confused ramblin' don't give you a headache. Just try to help that's all.
Johannes