I've failed their selection a few times now and am pretty sure it's the verbal reasoning that gets me each time.
It is their train set sure, but the first time I went for the assessment back in 2010 the day 1 then was the current day 1 and 2 combined (computer test am & interview/group exercise pm). I admit I didn't put anywhere near enough effort in back then for reasons other then just laziness, which of course I regret now (hindsight is wonderful). I did assume if you were marginal on one of the computer tests, you could perhaps still get the nod for the sim if you produced a good interview. Now it does feel like they don't give you a chance to meet the real person on day one, and you leave there having failed in my case with no one actually meeting you.
As Twinstar2007 said, it's unlikely a DEP can't operate an aircraft so the first few days are simply to see if you would fit into their company. People pass the computer tests with luck sometimes as much as some might fail due to bad luck. Personally I think that having the interview/group exercise combined on the first day at least gave you a chance to show who you are, and because of this failing then was far less disappointing then failing now.
ps. The most recent time I failed, I probably put two months of study into the verbal test and failed.