It's not necessary to mix everything on a single board just to answer that question In fact, mixing everything may not even make answering that question easier.

Relative priorities of projects may change. If I tell my devs "project X has priority at the moment" they just need to focus on project X's board.

In any case, if you colour-code you can see at a glance what task looks most critical by looking across a few separate boards.

In my experience (but maybe it's just me, maybe other people manage their projects differently), the type of task plays a greater role in its prioritization than the project. For example, we always fix bugs first, even if that pushes back delivery of new features. Stability issues are even more important than bugs. And security issues even more important than stability issues.

Yes you could have multiple boards, each with its own backlog ordered by priority, then use colours to see which tasks from which board are a higher priority. But surely the point of having an ordered list of tasks is to see which is the highest priority?

Yes fixing bugs first is sensible, but you still need to prioritise projects as well.

My point is that there is no need to lose the benefits of separate boards in order to solve the (non-)issue of quickly finding out what to do next.

And if you have a single board then try easily seeing how product x is going...

(btw, by colour-coding I meant features, bugs, etc. so you can immediately see what's what)