William, over on the Phaser Games site, has written a new component called UI Blocks which allow for an alternative to Containers. And explains in depth the difference between Groups and Containers in Phaser 3: "Coming from a background of flash I was used to creating MovieClips and adding other objects inside those clips. I especially use this for user interface components. In Phaser 2 I use groups quite a bit to create UI.
To understand the problem, we need to understand a bit about groups and containers. Lately, I’ve been seeing a lot of confusion about groups and containers. It is especially confusing for those of us that are coming from Phaser 2. Groups in Phaser 3 do not work the same as in Phaser 2.
A common question I’ve seen is ” why can I not add a group to a group?”. Let’s have a look at what happens when you add a child to a container or group."