The trick with aluminum is its modulus of elasticity (force needed to stretch it a certain amount) is about 1/6 to 1/3 that of steels and cast irons, which result's in Lee's recommendation that you tighten a sizing die 3/4 of a turn deeper than light contact with the top of the ram for their aluminum presses, rather than the 1/8 to 1/4 turn usually recommended for cast iron presses.
Mild steels usually have tensile strength on the order of 50,000 psi to 80,000 psi. 7075 T6 aluminum is in the same range. 6061 T6 is 40,000 psi-ish. But if you look up cast aluminum blocks, the tensile strength is down in the 10,000 psi range and they are not tempered. I don't know enough about aluminum casting to say exactly which alloys are most commonly used for something like a reloading press.
But it may all be a moot point, as reloading presses are not cast from aluminum given a T6 temper, nor are they injection cast from steel, but rather are cast from iron when they aren't cast aluminum. Gray iron can be no stronger than the cast aluminum block data I found. 10,000 psi-ish. The precipitated graphite can weakens it considerably compared to steel. Ductile ion can get you back to the mild steel range, though. Other than Dillon's mention of using ductile iron in their 50 BMG press, I don't know what the other presses being cast of iron are made from. There are also machined steel presses available, though not cheap, like the one Corbin makes.
Richard Lee has a photo in his book of one of his presses that has steel rod verticals and where he has turned the three of them down to 1/8 inch and can still resize a case, so steel strength levels are greater than are strictly necessary, as long as there is more aluminum in the frame than that.
I don't think aluminum is necessarily an inferior choice where thicknesses are adequate and as long as you allow for the increased ease of stretching. In that regard, the O-frame press is, in fact, a design that will keep die and ram alignment better through the stretch than a C-press design will. However, I have resized .308 cases in a Lee Hand Tool loading at the range before. It was hard, but it did it and the ammo shot just fine. I would not try to use it to resize once-fired brass out of someone else's gun, much less out of a military full-auto weapon that was fatter and/or longer than my own chamber leaves brass. I would not try to use a small base die in it. But brass fireformed in my own rifle and minimally resized could be made to work. Not fun, but doable.