On our last visit Splash was down for it's refurb. Did they widen the loading area during that time?? The way I remember it, it's not really much bigger then BTMRR. And there is a large, fairly steep hill that levels off just a bit near the actual loading area. However, I do remember the actual area/platform (not the hill portion) as being very uneven. My sister, who is the one in the wheelchair, had some major issues with the uneven ground here. She tripped one time and almost took a faceplant into the logs.
Our problem with BTMRR has been the narrowness of the platform. Since it's also the area where folks exit the ride and as more people in wheelchairs come in the exit to line up...it can get very congested. What the CM's have done to combat this is have the wheelchairs & their party, line up along the back fencing in a straight line. IMO, this has helped but not really solved the problem.
Luckily we've never had an ECV malfunction on us. Of course my sister is super paranoid of running into people or things so she'll usually get close to where we need to line up & then DH or I will talk to the CM and they will give us further instructions. So really...she and her ECV don't get too close to have an accident.