Look at Bourbon-Habsburg rivalry. We've had this discussion before.
For a social order to be inherently better one needs to examine the principles underlying the order. Piling on examples of bad monarchs -- particularly when examples of bad republics are close on hand -- does nothing to advance a theoretical argument.
You're welcome to make your theoretical model. Once you have, though, it has to be empirically tested. That's what the examples of bad monarchs are: counterexamples that you ought to take into account. If the opposing evidence is more than the supporting evidence, there's something wrong with you're model. I'm not an expert, so I'm not going to venture a scholarly verdict on Hoppe's hypothesis, but I trust I've presented enough grounds for questioning his theories.
The monarchist argument is that a king has no need to expand his power beyond the point when his kingdom is secure. He therefore naturally tends to govern as libertarians teach, minimally, and with protection of rights as primary concern.
In other words, the argument is that kings will behave as prudent investors, as bourgeois shareholders or rentiers. There are problems with that as well. Kings, like other rulers, are subject to influences and pressures. They are confronted with real or perceived threats to their power and react or overreact in response to them. Moreover, they are subject to lust for power, avarice, hunger for glory, luxuriousness, insecurity and all the other human vices.
Not all monarchs fit Hoppe's happy bourgeois scheme. Some kings have been brought up with an exaggerated sense of their own entitlements and a stunted sense of responsibility. One real problem was that the populace that the kind was theoretically reponsible to often had no voice in its governance. Another was that the members of that public didn't count for much in the thinking of the day. The desire for glory was long seen as the common corruption of monarchy. Flattering and deceitful courtiers have also long been seen as a real problem. It would be nice if monarchies corresponded to Hoppe's ideal, but it's not the case.
What Hoppe's doing is contrasting an idealization of monarchy with a caricature of republicanism. He's selling the mirror image of 18th century republican thinking. They offered a picture of republican leaders as public spirited and virtuous and monarchy as corrupt in large part because they had little experience of real republics in action. Hoppe peddles an idealized version of monarchy, that many who lived under actual monarchies wouldn't recognize as true.
No, the libertarian argument is not about the character of the king at all. The argument is that his position -- as one who governs by birthright and cannot be legitimately ousted -- makes him favor rights, legality, peace, and stability. It doesn't mean that bad kings are impossible, it means that even bad kings are nudged in the right direction by the necessity to avoid a revolution, while even good politicians are nudged into the wrong direction by the necessity to get elected.
There is a valid moralistic argument for Christian monarchy as well -- based on the fact that the Church is the repository of the moral teaching and installs the king -- but I did not make that argument in the post you are responding to.