OK, I've set up a test which I believe matches the situation described by DJPadre. All at 1920x1080P 25fps. Shutter 1/50sec, off (1/25 sec), OIS on and off. Road speeds between 70 and 100 Km/hr, various backgrounds from roadside vegetation to distant. Angle of shooting ranging from normal to travel (car beside) to along direction of travel (car ahead). Daylight. All hand held (no support). Road: bitumen. Wide angle to med on driver.
Padre, I do not doubt the unsatisfactory result you suffered, but I cannot reproduce it. Or, perhaps it is better to say, anything that seems to match your remarks. At 100 Km/hr close foliage is streaked as I would expect at these slow shutter speeds. Subjects further away (seen between foliage) are fine. I'm not seeing problems with rolling shutter or codec. What I see on the screen is what I expect to see. On still frames I do see the slew of the verticals caused by the rolling shutter and shooting close on foliage that will add to the streaking effect (at 100 Km/hr the world is passing at a rate of 0.55 metres per 1/50th sec). But anything passing slowly, or not at all, is sharp. Some sort of pilot error? Or the tests don't reproduce the original circumstances. I do think it very fair to conclude that there are no problems for hand holding.