Are CGM sensors like Ultrahuman worth it?

It's useful only if you want to bio hack. Gives usable insights for you to modify for whatever it is that you seek to bio hack for.

If you don't want to, its a pointless expense.

It depends. If you have a higher disposable income then yes.

Essentially, it does not have a high ROI but there is marginal RoI. So, something like this comes in the advanced stages of fitness journey

Helps if your HbA1c is > 6, simple

You can get idea of what it does from watching YouTube videos of other users and get to a broad conclusion if want one or not.