It’s been about over half a year (actually rather 10 months now… time is passing) since I bought the graveyard environment after facing it many times and personally viewing it as the most difficult/annoying environment to attack, and I didn’t regret that ever since.
For looking at your own base, I don’t like it at all (too depressing and hard to see, just as for raiding it; even though its visual quality isn’t any worse than the others), but that was clear to me before I bought it.
I have faced burning rocks often as well, and recently bought it for vouchers. I like the visual appearance, and it’s dark as well, but for me graveyard always was (also a year ago) and still is by far more annoying and “effective” than burning rocks, because the lighting of graveyard is considerably darker than burning rocks imho.
Christmas is relatively dark lighting wise (not as dark as graveyard though), but the environment itself is rather light, so it’s better so see things there. Some windows users reported problems seeing their cursor (blueish tone) on the blueish color of the dark snow environment in the christmas theme, but I can’t really judge that being an Android user with touch screen instead of mouse.
So for raw efficiency I vote graveyard.
Btw, might be slightly offtopic, but the only thing I like so far about the vouchers is that one can buy environments for vouchers with a “1 gem to 1 voucher” price ratio.
Needless to say, I recently bought all remaining environments for vouchers, thanks flare.