There are some limitations and drawbacks of Amazon ads' built-in A/B testing functionality:

Limited number of variables: Amazon ads' built-in A/B testing functionality only allows you to test one variable, such as the title or image. This can be limiting if you want to test multiple elements of your ad at the same time.
Limited number of test groups: Amazon ads' built-in A/B testing functionality only offers two test groups to test. This can be limiting if you want to test multiple elements of your ad and test more than two variations.
Limited testing times: The built-in A/B testing functionality of Amazon ads only allows testing for a limited period of time, usually between 7 and 14 days. This can be too short to see significant results, especially if you have a small budget and don't drive a lot of traffic to your ad.
Difficult to understand results: Amazon's built-in A/B testing functionality provides results that can be difficult to understand for advertisers without experience in statistics or advanced data processing. This can make it difficult to draw the right conclusions from the test results.
Limited flexibility: The built-in A/B testing functionality of Amazon ads offers limited flexibility to change or modify the test after it is started. This can be limiting if you decide to change your testing strategy after the test has started.

If you want to run a more comprehensive A/B test with more variables and test groups, it might be better to use an external A/B testing tool. This gives you more flexibility and control over your testing strategy and the ability to perform in-depth analysis of your test results.