I've encountered one merchant in Denmark using iZettle. He has since replaced it with the regular PoS. I can only imagine that is more expensive for him, but I wonder why he abandoned iZettle? Maybe it doesn't work that well? I am honestly wondering why I don't see more usage of iZettle. It could just be a Danish thing.
And yes, there is Swish/MobilePay, but that only really works if you're local. A tourist - even one from a neighbouring country - will not have access to that.