Does anyone have experience/opinions about using AWS Amplify?

Yes I used react-native and Amplify to built 2 ios apps "flash transcribe" (https://apps.apple.com/hk/app/flash-transcribe/id1610898677) and "landra".

Strength:

1. Really convenient to integrate aws services to your apps. I didn't even need to spin up an vm to host my backend or write code to handle auth but use cognito +appsync+dynamodb combo instead. You can setup a fully functional API backend with only a few commands. If you are familiar with aws services it will be a really powerful framework.
2. Infrastructure-as-code, ensure you won't break your backend accidentally and revert previous state any time.

Weakness:

1. Super slow when you have a lot changes to push and deploy to cloud every time.
2. Sometimes it could be a little buggy, i had to dig into the cloudformation templates it generated to troubleshoot sometimes. So at least some cloudformation proficiency is needed
3. Vendor locking of course. I never used firebase so not sure which is better. But I'm a heavy user of AWS so I don't mind develop all things there

Tldr: amplify is best for devops ppl who have some proficiency working with aws and IaC tools before and don't want to write backend code