I don't think hitting a target on a range is really that difficult using either - except maybe for women, and frail men. The latter two really ought to be in another trade anyway.
As for weight of ammo; with the lack of emphasis today on marksmanship and it's tactical incorporation, the average "combat load" is going to vanish very early in a fight. 5.56 or 7.62, regardless, there better be a motor transport resupply right behind you all the way to the bitter end.
Myself, I would rather have 100 rds of 7.62 and a suitable rifle than 200 rds of 5.56. The former is more effective against tactical barriers, and it will buck the wind better as well.
As for the effectiveness of those 20mm shells; it is worth noting that it is recorded as far back as the Boer war, then in two world wars, Korea, and Vietnam (and the rest) that even a thorough pounding with the heaviest artillery often will not have a significant effect on a well-dug enemy. Even less so in certain types of terrain. Some semi-exposed easily marked individual targets might be dealt with under ideal conditions.
Whichs leads to one of the achilles heels (and I think it has several) of the OICW. The "rangefinder". Laser rangfinders require a target with a certain level of reflection. A target hidden amongst what amounts to a visible expanse of foliage, unless it is in view itself (or there are other reflective features to mark it with), is not going to avail an accurate fix. Likewise over any expanse of forest, where your target might be located in depth, or very rocky terrain with no particular features conveniently at the target location. There are plenty of other factors that make the use of such gadgets impractical under many circumstances - like heavy rain or fog. And any optical sight is going to be affected in use with just water, ice, dirt, mud, dust on the outer lens surfaces. The specs say a 6X scope. I would be interested to know what field of view it renders at say 25, 50, 100, 200 and 300 yards.
And I hope they have considered light reflection. Any unshaded (any not *deeply* shaded) optical device with an objective lens will reflect light. There are some anti-reflection devices (like KillFlash), but they are add-ons, and cut the amount of usable light entering a scope. Without them, looking through a scope can be extremely risky business. Even if the sun is behind; take the head away from behind the scope, and (unless OICW has a shuttered eyepiece) a telltale signature visible for a long way will be produced.
I seriously doubt that the gadget factor will be enhanced in extreme heat or cold. Even the best batteries won't last in extreme cold. Often they are not even reliable under ideal conditions.
