When I was back at an automotive startup, we had to write an integration with ADP's DMS. I can say without a doubt that it was the worst API I have ever had the misfortune of working with. Horribly buggy, the documentation didn't match the output, endpoints disappeared at random, different endpoints required different auth keys. Made no sense at all.

I even remember a time when our ADP representative called us and said she thought the project should be called off due to the amount of malformed payloads we were sending her.... on our development account. Not production.. Not even testing. Development.

ADP has left a bitter taste in my mouth since then.