New theme integration shouldn't have any issues because of all major changes implemented via custom mu-plugins and ACF plugin and current theme response only for the templates (pageview).
- custom Posts declared in mu-plugin "register-entities"
- custom pages declared via custom template in the theme
- custom page data provided via ACF plugin and meta fields which declared in mu-plugin "register-entities" (ACF Docs, Meta fields Docs)
- pantheon recommended plugins (Pantheon Docs)
- key management provided via Lockr plugin (Lockr) and keys defined in mu-plugin lockr-keyring.php
- theme required modifications stored in themes/"theme_name"/app
Known issues
Pantheon integration known issues: https://pantheon.io/docs/modules-plugins-known-issues/#wordpress-plugins