I heard this recently. With SXA it was not possible to limit the usage of components in certain placeholders. Well, that is not true. With SXA you can do this by using placeholder settings. It is located underneath the Presentation node. You can simply add a new item under the Placeholder Settings item and configure for which placeholder it is applicable and which components can be added.
While this works perfectly, you will notice that this is not applied to the sidebar. It will only work when you use the Add here button of a placeholder. And it makes sense because the toolbar has no placeholder context.
But this way you can limit which components can be used in which placeholders. Very useful!
A second way of using placeholder settings is to rename the default names of placeholders. You can create placeholder settings for placeholders with descriptive names like “Main content” or “Left column” and it will make much more sense to your marketers. You don’t have to configure anything else for a placeholder setting. Just give it a name and fill in the placeholder that it is applicable to and you are done!
It still amazes me sometimes of what you can do with SXA.