I haven't used it on oak - softwood and lighter hardwoods only.
The size of the chisel will make a huge difference: the force required to drive it will go with the square of the radius (I think), so that's probably the biggest issue. A bigger motor would be the thing, and a longer lever for the chisel. I'm pretty sure if you're making lots of mortices more than 3/8" in oak (in one go), it would struggle, and yes, you'd need one with a bigger motor.
I've used it up to 1/2" without problems, but I was doing 3/8" in softwood the other day with lots of jamming, because the resin was gumming up the chip ejection. Re-adjusting the auger clearance (to be a bit less than normal) pretty much fixed it.
As for those faulty motors: perhaps I wasn't explaining - it seems to have been a duff batch of wire in China, supplied to the motor winders, who made for a number of customers, including the people who make Axminster's (and other people's) mortisers. The other one I had that failed was on a Dakota (Rutlands) wet grinder. Worked fine one day, then smell and no rotation.
It's not uncommon for a component supplier to mess up, and that motor fault is the sort of thing that would be hard to see coming. You could only test for it destructively (well, shock+vibe, perhaps), but given the relatively small production runs involved it would be a very expensive thing to do, if costed per unit.
A good example of a similar problem from an entirely different industry, is the power rail smoothing caps that were supplied to almost all computer motherboard suppliers about ten years ago. They used a flawed recipe for the electrolyte used, causing them to explode. It affected pretty much everyone, including Apple and loads of PC makers.
In the case of Apple (whom I'm only mentioning because I can't remember the other makes for certain), they extended the warranty on their eMac model to take the problem into account. We had a machine fixed (for free) that was at least three years old and well out of normal warranty. I repaired a similar one myself, and I know it would have been at least an hour's labour plus quite expensive parts. Other manufacturers basically told their customers, "tough!".
My point: sometimes you can't see a problem coming no matter how much QC you do, and anyway the law of diminishing returns applies - the testing would make the product too expensive!
In my case, Axminster handled it well, their after-sales exceeded my expectations, the problem was fixed, and I have every reason to believe they read the riot act to whichever Chinese supplier it was.
I don't read that incident as indicative of an ongoing quality problem, as Axminster had root cause diagnosis and a quality improvement process in place to feed back problem data to the OEM supplier. In fact, I'd be more likely to buy from them again on that basis.
Hope that makes sense.
E.