This is why TDD exist, it doesn't necessarily improve the product or delivery speed, but it keeps you sane while the project is half baked.
I've found incorporating tests on my personal projects allow me to pick it up after even a year or two, but if I don't have tests, they inevitably become so jumbled that they cannot be salvaged once the initial effort is over.
tests mostly work for technical code, it would be harder to pick up gameplay stuff.
Though that's why open betas exist I suppose, but for most studios that want to keep things under wraps, paying a lot of testers is extremely costly (and thus buggy releases).
