This is relevant because Western monarchs claim the office of Rex/Regina.
Unelected officials should never have significant power. This led to a harmful coup d'etat in Australia. It's not a ceremonial office, and simply the fact that the emergency button exists makes it possible for perversions to continue and cause serious issues (and it has).
Partisanship is very far from the only cause of dysfunction in governments. I'd argue it's not even the worst.
However, even if it’s generally accepted that the child or close relative of the current leader should become the new leader there will eventually be back and forth over time between the royal family and other members of government. In theory they might have absolute authority, but in both practice and tradition there will be many informal limits.
In general, monarchies pretty much have had the absolute maximum of power concentration possible without disastrous consequences.
Nonpartisan in the sense that the Queen is nonpartisan. So of course, partisan. Still, nonpartisan like not publicly wading into political debates, and not opening up the office to the vulgarities of electoral politics.
For example, Germany elects its president by convening both the Federal and State legislatures together, who then vote. By pulling in all the regional parties and legislators, the President has to have broad support across party lines. It's also convention that the President doesn't wade into daily political issues. The result is a largely ceremonial office, usually held by a broadly respected statesperson with former government experience. Which is what you want for the "emergency button" you hope you don't have to push in the first place.