What helps the Milwaukee is that the o-ring is thick, about .25" cross section I believe. Most aluminum jacks use smaller section O-rings that wear faster and/or are of a lesser quality. Still, in theory, it would be possible for the Milwaukee o-ring to twist in its groove, even in a rod application. That it does not do it has to do precisely with the thickness and perhaps hardness of the o-ring. Also, the polished surface finish on the ram prevents the o-ring from twisting. In general use, and not specific to any particular pump, if a ram, or piston, surface becomes rough for whatever reason, like rust that was sanded and the ram was not polished afterwards, there is a high probability that the o-ring sealing the ram will twist. Now, the increased roughness may not be felt as being too much by a person running its finger over the ram, and in fact, the person may think that it is good enough, but the o-ring under pressure will sure feel that extra roughness and twist because of that.
On the other hand, an u-cup should not twist in its groove because of its shape; they are so much harder to twist. The Milwaukee o-ring could be replaced by an u-cup for rod application and it should work as good if not better than the o-ring.
It is true that many cylinders use O-rings as main seals, and have done so for many years, more so the lower priced cylinders. Just because they are used a lot does not mean that the O-rings are the better seal, it only means that the O-rings are cheap and work well enough and manufacturers can get away with using O-rings. But better quality cylinders usually use better seals. But usually the cylinders used commercially are repaired on a regular basis, so they operate with seals in better shape. Of course, there is always the cylinder that does not gets overhauled until it leaks like a sieve, but that is lack of a preventive maintenance policy.
Cylinders used by homeowners, on the other hand, are used for many years with the same seals and are only changed, reluctantly, when the cylinder does not works anymore or leaks like a sieve. And even then, some owners still want to use the old seals and only replace the worn one, even if they are over 10, 15 years old.
So, even though O-rings are used in many cylinders, because they work good enough, probably are cheaper to manufacture and the O-rings are cheap to replace, I still don't like its use as a main seal. For that application, an u-cup would be better but more expensive. When I see a cylinder or a jack pump using an o-ring as main seal, I think of that pump as being a lesser equipment. It is like a high performance sports car using bias ply tires; simply not good enough.
That is what I was thining. I'm far from a jack expert, but me has a feeling that is a rebaged and price inflated unit. 
I'm sure Hiball will be along soon to sort this out. 
