The basic premise of the thread assumes the Middle Ages were a lot more modern then they are. The modern age is full of formal, written constitutions that clearly limit and delineate the powers of individual leaders, and have court systems with authority to rule on what ambiguities remain. In the middle ages there generally was an idea that customary law and arrangements, religious mandates, or (in some contexts) the Roman legal tradition could sometimes, limit the laws and powers of a king, but where, when, which, and how these governed was all pretty amorphous, and fundamentally just came down to rhetorical arguments in a system where real power was about who held loyalties and how effectively they could exploit them. Partly this stems from the fact that medieval people thought that in their whole tangle of laws, customs, kings, and traditions there was the hand of God pulling the strings, and hence that there was some correct way for things to be beyond whatever legal arrangement man had happened to have settled upon. Hence if one heir was manifestly more suited than another the medieval mind was willing to accept that some "authority" must support their claim, and it must be the correct one.
In Westeros the tradition of the Iron Throne was male preference primogeniture, possibly strictly male primogeniture but even that had not yet been fully tested (can a daughter succeed instead of a brother?). This sort of amorphous succession was actually pretty common in the medieval period. In House of the Dragon we have a King trying first to clarify, then to set a new succession order. At first he is just clarifying, establishing that, with no direct male heir his daughter can inherit, which is settling an as yet unsettled question of the succession laws (something he is almost certainly a king of sufficiently respected power to do). But then when male heirs emerge he stands by her claim and insists on strict primogeniture over male preference, which is a clear breach with tradition, yet still possibly within his power, and one must be careful when one questions a king. While this latter choice is one virtually no European nobles and monarchs made, it is not completely beyond what would be considered. The circumstances of her already being long established as heir, being an adult groomed for the inheritance, being married to another claimant, and the alternative male heir being unsuitable makes it substantially more believable. But ultimately the king's power over his own succession is questionable, has unclear limits, and really comes down to what people are willing to do after he passes.
So to answer the question, I prefer a society without this fundamental ambiguity over what the succession laws are or how they can be changed. Having a clear rule is more important than having a good one, because all systems will give you sizeable helpings of great, mediocre, and terrible leaders, but ambiguity also gives you lots of succession wars between them.
For fiction and roleplaying games, however, I prefer the messy medieval succession ambiguities because they create lots of plot twists and interesting and/or gameable content.
