I spoke to a dealer about this when I was tractor shopping and they recommended to me that in the size of tractor I was looking at (well under 40hp) not to use a splitter. They said that the use of a splitter is just too much for the hydraulics of the small tractors and the fluid can overheat and cause damage as well. They said "bigger tractors" are ok, but I didn't ask how big they meant by "bigger tractors," but I have always assumed that 40hp and up should be no problem. Wish I knew more, lol.. They recommended 'just go out and buy a nice Honda powered one. They're not that much more expensive anyway." Thing is, with the price of fuel, if you're using a much larger tractor, it would seem you're going to go through a lot of money in fuel, and STILL might be better off with the standalone unit, despite the extra storage space and maintenance.
I think there is probably somewhere a sensible limit. I mean, I still think I could probably get away with splitting 3 or 4 cords a year (what I normally use) with my little 26 hp. tractor, but would likely be pushing the envelope if I were to do 30 or 50 cords a year, like part time firewood sellers probably do.. Anyway, I'm still doing it by hand, and haven't even been doing the 3 or 4 cords a year, mostly just buying the wood. $1500 for a splitting machine buys quite a bit of firewood.. and is no maintenance.