Passed the 2018 version of the Amazon Certified Developer Associate exam yesterday. I used the courses on acloud.guru and LinuxAcademy to prepare, and the LinuxAcademy one is definitely better. I also used the official practice exam and the WhizLabs practice exams. The WhizLabs ones are helpful, but the official one isn't worth $20 IMO.
The test itself wasn't terribly difficult, but the experience left something to be desired. I took it at a remotely-administered kiosk, and when it came time for the test, the guy there spent ~15 minutes on the phone with the test administrator so they could remotely reboot the kiosk (the previous test-taker had to leave due to technical issues). The kiosk crashed 3 times while I was taking the test, but it managed to stay up long enough for me to finish.