> The only exception I have to this is S3. There are so many S3 compatible providers that I don't see any problem with using S3.

I've strongly held this view in the past but after recently being on a project where we trialed using other providers, there's a lot missing compared to the AWS S3 offering that has made me rethink this stance a bit

Permissions being a big one, many providers offer little to no fine grained control. With Digital Ocean for example it's all or nothing, you can only generate access keys that gives full access to all buckets[0]

[0] https://news.ycombinator.com/item?id=37724393