APIs or services that cause problems when testing?

I have heard from my clients that when they integrate with FedEx services and test their applications with FedEx Sandbox, it causes testing issues 2. The test data is not available, services are slow to respond, and intermitently not available. This means that testing typical scenarios sometimes takes days instead of hours.

What other services or APIs case your problems during testing and why?

I once hat something similar with with the PayPal sandbox.
The availability and data was fine.
But we accessed it via Browser automation and they changed the UI from every 1-2 month.
Was not a big hassle, but on regular base our automation coded need to be adapted to match the new DOM locators.
I did not research, but I wonder if there was way to get informed about new deployments.
It always surprised us.