ArM's rulebook has not been criticized enough. Not the rules, the book they're contained in.
Case in point, magic resistance. This is very important, right? This is something that's going to come up on a regular basis in a lot of games. So let's open the book up and try to remind ourselves how it (and spell penetration) work.
The actual calculation for magic resistance is hidden within three paragraphs of text under the heading Magic Resistance on p.82, with nothing to signpost it. Same with the effect Parma Magica has on this score several paragraphs later. Any sane author would include this in the character creation rules, as that's when you're going to initially work out what your character's magic resistance is, but no, they're in the Hermetic Magic chapter, a categorization that isn't very helpful from a rules perspective (particularly when there's a DIFFERENT chapter for Spells). Any player that hasn't dutifully read through this whole chapter of the book, which they probably haven't at this point, isn't actually going to have any idea what their magic resistance is.
Can I find this section on Magic Resistance if I need to remind myself? Not really. The Table of Contents only lists full chapters, with no page references for headings at all, so it's anyone's guess. The closest you can get is checking the index in the back of the book, and of course it's entry on magic resistance lists every significant mention of the term- pages 85-86, 87, 183,
184, 189, and 191. What you're looking for is on p.85 but there's no indication of that anywhere.
Guess what isn't referenced at all in the Magic Resistance section?
Penetration! Not once is penetration, the one value which directly interacts with Magic Resistance, mentioned. This is obviously because penetration is explained slightly earlier in the same chapter, but from a reference perspective, this is a mess. Finding out how Penetration works, of course, runs up against the same problems as Magic Resistance if you're just searching for Da Rules. Right on down to how Magic Resistance and Penetration interact with each other, probably the most important part of all this. It's only brought up at the very end of the section on p.82.
I'm currently starting the process of transferring RoP: The Divine to Project Redcap, but after that...
In theory, PR includes rules reference pages, which would be massively helpful, but they were written long before the Open License so they don't actually function as rules references at all. When I'm done with this current project of mine I'm going to look into helping to fix this. It should be possible to learn how to play ArM without feeling like you're hunting information for your college finals.