DI is easily one of the best games in the last decade, but I have to admit that the main multiball is too hard by design. First off, it's only two balls to start and it takes a lot of hits to add a ball. Then you've got multiple active deterrents running at the same time - the playfield magnets, the moving QED guy, the flipper disable shot after only two jackpots, and finally, the playfield design itself doesn't promote keeping more than one ball in play very well. I know it's supposed to be a "battle" but it's just demoralizing how short it ends up being.
Of course, all you need to do is look at Pat's WMS-era games to see this is all intentional - he's always made multiball hard to get and insanely difficult to score in or last any length of time. He just doesn't like multiball.
Contrast with Iron Maiden, the new girl on the block, which I find strikes a nice balance between overly-multiball heavy (Hobbit) and DI, where the number of them and more importantly the duration is kept at a more satisfying level.
I don't think it makes DI a "worse" game, just different. Personally for me it's less satisfying, and it's not something that could be fixed in software because it's a conscious design decision. Or maybe I should just play better.