Models of devices sold in different countries can vary in some subtle but important differences even if they look the same on the outside.
The obvious one of course is the Z wave frequency. But power levels are often different, in some cases mandated by law at different ceilings.
My understanding is that the only devices that are on the official compatibility lists are ones that have actually been tested by SmartThings staff for compatibility with the hub for that region and the other certified compatible devices. This can take some time.
So in some cases it may be that they haven’t yet done the certification testing. But in other cases it may actually be that the device from one region is not compatible due to engineering differences in the models.
Harmony Integration
As far as harmony goes, I don’t know of any specific differences between the European and the US models, but there is a bug right now preventing authorisation of many third-party services with Smartthings UK accounts. that one is definitely affecting harmony use. You can track that issue in the IFTTT topic in the UK category.
Once the authorisation issue is fixed, it’s probably best to go either to the knowledgebase articles or to the FAQ topic. The Harmony integration was very popular and went through many different versions and states before final official release, which is why just searching the forums can be confusing. But the FAQ topic is short and succinct on the current official easy method.