I've split a bunch of yellow birch over the years and its usually stringy stuff, often times the splitter will go all the way through it and then I still have to chop the stringy stuff apart with an ax.
Aspen is a dream to split usually one whack with the maul and it splits and is ready to go, spruce and pine not so easy to split. But as long as I can lift the 40" long pieces I don't split them as that just means they will burn faster.
I burn about 10~11 cord a winter with the boiler no matter if its -10 or +10. I have tons of aspen available around the farm so that's why I burn it.