Hey, great writeup yourself! I can understand the desire to validate state. The disadvantage to moto is the same disadvantage that the original boto library had: it needs to be constantly updated to match new API functionality. boto3 solved that (along with all the other AWS SDKs), and Placebo similary relies on boto3/botocore and the APIs themselves, so it doesn’t need updating.
If you’re mostly using long-established services, there’s no problem! But I find that adopting serverless means being ready to take advantage of new services whenever they appear, so I like the Placebo approach :-)