Yup!! I've been ranting here about Unicorn dragon many times. Crusader is also ultra rare but you know you'll get it eventually because red doesn't split. I got two if them while trying to breed unicorn. 150 fails now, only two times diamond didn't split. But of course virtue did.
According to this, I assume the chances of getting unicorn/dark angel/helios (haven't tried dark angel/helios yet but black and mythic split too) is less than 0.5. That's assuming after 200 tries I still don't have it. Plain tedious and, like tonguelashing pointed out, virtually impossible.
It's such a turn off!