Does it actually work in mainland China, with the Great Firewall and stuff? Will I be able to send iMessage or Facetime, check maps etc.?
Airalo in China uses China Unicom , and Apple ensures to stay compliant with Chinese Govt so their services continue to work . So the short answer is yes all the service you listed will work without issue. It’s mainly that apple themselves ensures this continues to work more then anything to do with the network .
If you see here apple lists chins Unicom as supporting FaceTime over cellular . As well as iMessage , so you should be fine ! 
If you give Airalo a shot please use my code: ZAHEER3923 for 3 off .
Im in China now, using airalo china esim 20gb (china unicom). Im able to access everything without vpn. Got freaked out as i know this is not possible. Scared the bat out of me withoit using vpn since i dont want to get into any trouble. But seems like the explanation of airalo is good. No need for vpn then, that means faster connection, and cheaper too since you dont need a vpn
No issues using WeChat or Alipay with Airalo for anyone, right ?