is it safe to chmod 775/664 with owner www-data:www-data?
Solution 1:
Usually you just want to have upload folders or autogenerated files to be writable by the www-data user.
Anyway, the risk you are opening here is that if your web application has any bug/vulnerability that might allow an attacker to execute code on your server, this code will execute as www-data (the user the apache process is running) and it could completely delete all your websites.