It's a question of how much you can do with the tool. If Lambda only supported, say, pure Python or Node libraries you'd run into cases where you couldn't use it or would see big performance penalties. Being able to push a container with, say, a high-performance image processing library which your Lambda needs makes it a lot more versatile but also forces you to think about things like x86 or ARM.

Overall, I see that as a win: you can get started by putting code in something like Python or JavaScript and not caring but when you do hit more complex dependencies you have a path which is an extension of what you've already been using rather than having to switch to a completely different service like ECS.