This is actually utterly disappointing. I was awaiting the guesting very much, but not only to guest on other EU servers but also on NA servers, since I have many friends in both regions and we actually wanted to play all together. Regardless of the technical reasons or the instability of the connection between data centres, I’d appreciate a warning pop-up being thrown that “guesting on other regions might be instable” than this absolute restriction like this. (referring to the statement, that the guesting would be way too instable if allowed cross-region and they want to ensure a stable and fluent gameplay)
I hope that this policy won’t be kept constant and some workaround will be added, that people from all regions can play together.