Last time I checked (a few months ago), Planner still could not be backed up. Like, at all. If someone went in and deleted a whole bucket you can't recover it, not natively, and not with third party. So that's a big fat no from me.

I seem to recall dumping Planner to JSON easily enough. I don't know if you can easily restore directly from its JSON, but the JSON was nice enough to work on for what little I needed to do with it.

My current employer shut off Trello and forced us over to Jira and is threatening to disable Planner, so I'm "not allowed" to rely on Planner enough day-to-day so it's possible it is either better or worse than I remember it being in that department. But this Jira outage has me reevaluating, and they haven't turned off Planner yet.

You may be right. Planner does have lot going for it (if your company is already in 365).