Rootstock choice is the number 1 reason a citrus tree is hard to grow in some areas. Most propagators graft to the easiest rootstock, not the best.

 Maybe you need either a graft of flying dragon  or a hybrid with It if you Are dealing with northern Europe  but I'm no expert

What do you mean by difficult exactly ? Difficult to grow the plant? . . . or difficult to get good fruit?

I have two Shiranui plants. One I purchased and I have no idea what stock it's on. And then I grafted a piece of it to a citrumelo rootstock. It has done very well and is loaded with fruit. But the fruit are not the tastiest yet. Maybe when it gets a few more years on it the fruit will improve. As far as difficulty in growing the plant, it is vigorous, but no different in culture than any other citrus.

I read it should reach brix values up to 21. I also look for pollen of this cultivar.