I had to rewrite our entire scheduling system at work to use Outlook instead of Google Calendar. The guy who wrote the Google Calendar scheduling system made it so unmaintainable that it was faster to just rewrite the entire thing from scratch (1000+ line lambda function with almost 0 abstraction).
At least 90% of what I wrote is just exception handling. There's ~15 different 4xx/5xx errors that can be returned for each endpoint, but only 1 or 2 200 responses.