no spam possible

Only contacts can message you. Yes, it technically almost eliminates the possibility of spam, but it also means you explicitly have to exchange data first. So, no "here's my email address, you can send it to me there", no "do you have XYZ's address", no "I lost access to my previous address, this is my new one". I realize all of these scenarios can be seen as risks, but they're still important in day to day life and will be a major hindrance to adoption.