I recently added achievements from Google Play and login to the game.
Didn't encounter any significant problems.
The only inconvenience is that the authorization key is created for Android, and you have to test it on a smartphone; web testing doesn't work.
I know that for web testing services, you can create a separate key for the web, and I did that a long time ago, but this time I was too lazy to do it. I don't understand what could have gone wrong for you, because in the Construct 3 project, you only need to add one event to request login.
To create a key, you need to use the fingerprint of an already signed application, which is very easy to copy in the developer's account, where there is an “application integrity” menu.
There are no problems with plugin version incompatibility either; version 2 services work fine.