Personal unions were countries or states that were legally represented by a monarch. This could be as you stated, Hungary and all its constituent lands. Or Austria, or Brandenburg-Prussia, or when Henri IV de (de Bourbon) Navarre took the throne of France (leading to a personal union of northern Navarre). Personal unions were not a single country. I study Brandenburg-Prussia, particularly after the 30 Years' War, and nowhere in any sources are these lands considered one until after the formation of the Kingdom of Prussia (which could be its own topic). But let's take a moment and ponder: why weren't these lands considered one? Wouldn't they be ruled by the same person? The same council? Wouldn't they have similar or the same laws?

Not at all. The only tie in a personal union is the individual who has inherited the lands. In many (if not most, or all, cases) councils were established from local nobility to oversee the lands added to that dynasty's rule (see the Privy Council or Geheimrat of Neumark and Preußen after they were acquired by the Hohenzollern dynasty, namely that local nobles were formed into a collective council that oversaw local infrastructure and development while the monarch administered other lands [in this case Brandenburg, and parts of Kleve, Geldern, and such depending on the era.]. The monarchs would consult with the nobility and relied heavily on their opinion, their knowledge of local laws and customs, as well as their knowledge of the general customs of the area.

Other examples include Great Britain and Hannover, where Hannover and Great Britain were unified by a dynasty yet acted separately for their own interests. After all, it would have been a hard sell to have nobles, with great lineages in Hannover, suddenly serve and exert all their influence for the British crown when Hannover itself did not necessarily benefit from such a union (like the numerous times Hannover was occupied or fought in continental wars the British Isles were able to avoid due to their navy and commendable admirals). From my own reading of primary source documents, the Privy Council's would meet and acknowledge the new ruler's legitimate rule through seeking confirmation of policy: we will tax this much, build these churches, and maintain these bridges. But barring any special case, particularly of notable rebellions or unrest, the rulers seemed content to let the local nobles govern so that all interests could be met.

Imagine you are a ruler of a population of a 6 million. You have a history of people that know tax, commerce, local trades, problems, languages, religions, cultures, and such. You get word you have inherited a distant land of about a million and a half people. These people have entirely separate languages, cultures, religions, and different (but similar) tax and commerce laws. You are already managing a large and diverse land. This lands is beset on all sides by problems of religion, legitimacy, prestige, honor, welfare, stability, and general peace. Now you have a potential thorn in your side, but also one that could be a boon. At least in the case of Brandenburg (Prussia), and Britain (Hannover), a large amount of autonomy was granted with oversight, by granting the local lords continued exercise of powers so long as they recognized your rule.

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Personal unions were most definitely tied through each figure-head themselves. Brandenburgers and Prussians were not united until laws were enacted that exerted influence on them all. Until then, Brandenburg was ruled as Brandenburg; Prussia as Prussia; and Kleve, Mark, and other Westfalen lands as Kleve, Mark, and other Westfalen lands. But Prussians did not consider themselves part of "Brandenburg". Those were other lands, with other laws. And I'd like to add that both of my examples, of Brandenburg-Prussia and Great Britain-Hannover, take place after the Renaissance. Personal unions were exactly as stated by the name, lands tied to a person in a union, but not to one another.