I split quite a lot of wood, and in quite a range of types, from reasonably easy (straight grain macrocarpa) to very difficult (large old falled bluegum with twisted grain).
These days I mainly use a fiskars X27, which is a very nice splitting axe (not too heavy, fast swing, weight in the head, does not stick).
I also use a maul at about twice the weight, but mainly to releast the x27 on the rare occasions it does stick.
http://www2.fiskars.com/Gardening-and-Yard-Care/Products/Axes-and-Striking-Tools/X27-Super-Splitting-Axe-36#.U1WY4FdEBdU
In a side by side comparison to a good hydraulic splitter, against myself using two tyres on chopping blocks and a helper to load/upload them
(and two people on the splitter) I am faster for the first hour, then slow down on easy and medium wood, splitter wins of difficult wood, but only the
very difficult (twisted grain, lots of knots).
Knowing how to use the axe, using a tyre to contain the wood, pacing yourself, and using the twist on impact technique at the right times makes
a huge difference to axe splitting.
I would love to try one of these axes, unfortunately hugely expensive so not worth it - keep meaning to try asymmetric weighting on one of my axes to see if its a gain..
there are videos of it being used on more tricky oak, and discussions by 3rd parties that would indicate it is still useful. if you think any splitting is dont by cutting through the wood then you need to spend more time behind an axe.. the theory of operation on this thing should scale through most wood - but of course difficult wood is always difficult, its just a matter of scale..
And I am not big (140lbs...), so many axemen would do MUCH better.