That's something of an exaggeration - most things will still work perfectly well. After all, there hasn't been a vast amount of development in either branch in terms of updating existing modules and only a fraction of that development will have been divergent to such an extent as to cause failure.
However, the main problem is in all of those odd little edge cases. ROL would be mad to endorse a pick-and-mix solution because where would their support obligation to the user end?
And as the ROOL staff work for free, our support position would be no different to today: you get what you're given.