Look, I get why bending or breaking the rules you've set for your own magic systems is fun. Some of my favorite moments in Fate are a result of that. But what I'm getting at is that the rules are so frequently bent, broken, or even retconned that it's hard to take anything anybody says seriously anymore. If these are just guidelines and not rules, then why does Nasu go into such extreme detail into them and explain every minute facet of his magic system even when largely unnecessary?

You can't just hand-wave it as "Oh well, the characters aren't omniscient, so it's fine". I've seen this excuse thrown around a lot and I think it's a terrible one. From a narrative standpoint, you can't have exposition dialogue be wrong most of the time, especially when the character explains things with a tone of objectivity and certainty. The audience has no reason to believe otherwise that this character's words are misinformed. And if they do, then they would have to start suspecting the explanations of every character such that nothing really adheres to any rules at all and explaining how everything works becomes pointless.

In FSN for example, Rin is the exposition machine for a large portion of the VN. She clearly states that Command Spells are non-transferable and even reinforces this statement by using it as a logical premise to deduce the identity of Lancer's Master. The reader therefore has no reason to believe that she is wrong as she says it with a confident tone with certainty and then uses it to make an accurate deduction. Then, in Fate Zero, the transferring of Command Spells is featured prominently in the story. If Rin had said it in a less certain tone, like "You probably can't transfer Command Spells, not that I know of" then yeah, the "mystery roots" would be planted, leaving it open to subversion and rule-bending later on, but it wasn't said like that. "But Rin isn't omniscient, and it's in her character to act confident like she knows everything!" Yeah, I get that it's in her character. That still doesn't mean I shouldn't believe her when she said it, because if I didn't, then I shouldn't believe what anybody says in any Fate-related media ever, no matter how smart they seem or how confidently they say something.

This is just one of the things I don't like about Fate in general. Either don't try to explain everything in extreme detail at all and just have it be a very flexible soft magic system, or go all in and adhere to the very rigid rules that you created, maybe break or bend them every now and then but not most of the time.