I’m going to lay out a typical scenario, and then how it should work. I cannot say how it does work. That is impressively opaque.
On my warrior I have multiple talent loadouts. I start on loadout A and get all my bars and buttons in the places I want them. This loadout does not have shared loadout checked. I switch to loadout B (same spec), and everything changes, including hotbar positions and what abilities are on which keys.
That’s fine… but it is ABSOLUTELY CRITICAL, that you give us an EXTREMELY STRAIGHT FORWARD way to declare that I want to copy the EXACT configuration from loadout A to loadout B AT ANY TIME.
Note that there should be ABSOLUTELY NO PRECONDITION for this action. There isn’t even any obvious way to figure out wtf your “shared action bar” configuration is or which one of my loadouts is setting that (if any???).
The way it currently works is, there is some crazy obscure way in which if you do things in a very particular order, you can managed to sort of get a shared configuration. It’s objectively TERRIBLE. I am certain the way it works can be explained. That does not change the fact that it is wildly unintuitive and painful. The system needs a fix. Just do what every addon does and make it straight forward to copy from one loadout to another.
This is also one of the reasons every M+ streamer you see change specs never actually uses loadouts. They manually change their talents because keeping consistent ui configuration between loadouts is absurd.
Edit:
After an unreasonable amount of effort and much redundant work, I’ve determined that if I manually check shared action bars on every single loadout I ever make RIGHT AWAY, I will have consistent action bars whenever I make a change on any of them. If I forget to check that box it will be an awful mess, specifically because there is no copy functionality. I realize now why you chose to make shared action bars opt in. It is a hard decision, but the real issue is not having copying. Copying on its own should have been the first iteration. Shared action bars on its own is just worse in multiple ways.