With passenger manned exits, the likelihood of the exit being opened in panic or adrenaline-fuelled and before being instructed to by the crew is high. The evacuation checklist and procedure doesn't include this and has to be altered rather promptly when and if it becomes apparent that the overwing is open and the engine is still running at idle.
There's a lot of disdain for passengers standing on wings and stating how easy it is to slide down the flaps when they're set at 40. Hmmm. I beg to differ. If it's on fire, then sure you'll get down somehow but it is "somehow" and how agile and prepared are most passengers (or flightcrew for that matter)? Have a look at the height of the wing and flap extension on your next walkaround, why not extend the flaps to 40, ground activity permitting, and look at the distance to the ground. Also look at it when you next pass the exit on your way through the cabin. It's not comparable to the "ease" of a slide and slide evacuations always cause some injuries. How about a wet wing, an upper surface iced wing, a de-iced wing with fluid still extant? This came up in the Glasgow evacuation and it's more valid than some give it credit. The other aspect is that removable overwing exits are actually fairly heavy and cumbersome and where does that door actually end up when it's removed? The -800 exit is a thing of beauty in comparison, but this wasn't an -800.
So how do we ensure that all passengers follow all instructions? Successful evacuations are controlled evacuations.