I have no direct experience about longhaul Y special meals, but one experience from shorthaul J. I ordered a gluten free meal just for fun, and what I got was a tray of superbland, inedible trash. Not even the FA could figure out what it was, just that the meal was also lactose free, vegan etc etc. This is one of the very few times if not the only time that I didn't finish my inflight meal.

My advise: if you don't absolutely need a special meal, stick to the regular options. Those special meals are made terrible for a reason; they're a lot more expensive to prepare, so airlines don't want to encourage people to order them.