Yes, would be interesting to know how often it fails for experts like Rob, and also for amateurs.
My guess is that the risk increases with the size of the infusion since it takes more time, and there might be many hoses, clamps, buckets, etc, involved in a big infusion. And you have manually time these clamps/buckets and make sure the inlets doesn't breath air, I can imagine. So if you forget one thing...
Rob, regarding buying Finnfoam, I received one reply when asking if they can supply it. They said straight away that they could sell a custom a pallet of 10mm FF700, or any desired thickness. But after I told him I'm in Sweden, he hasn't replied for a few days. I will try to call him and see.
But I believe it is like eruttan said, that the brand of he XPS isn't important. It's only important to get the high strength/density foam. When the foam is produced, there are probably just have some knobs to turn to increase the density of that batch. So any XPS factory should be able to make it. You could maybe even ask for higher density than their highest grade, if they are making a custom batches.