e.g. Arms law tables are counter-intuitive in cases in which a heavier armor is worse than no armor at all (soft leather vs at1).
Agreed, but this is different than being rules-heavy.
e.g.2 Character law & Campaing law. Try to explain a rookie the hodge-podge that is character development. "Normal" Skills, armour, spell develpment, body development and the weird attribute-less ambush skill (including its specific resolution mechanic).
I agree, no doubt RM could have benefited from streamlining, like RMSS tried only better. However, that's still only a handful of different different mechanics for the skills, far less than most other games that have no unified central mechanic. The vast majority of skills use the same central mechanic in RM. Open-ended roll + skill/mods - difficulty. In fact, many other systems use this same mechanic.
e.g.3 Character law & Campaing law. And what about skill resolution? What kind of target number is 111? Why some skills, which are developed in the same way, have a target number of 101?.
There were countless of nuisances that were neither polished nor streamlined in that ancient edition, and I'd say that not only makes it rules-heavy, but also complex.
At it's core, RM succeeds if you get 100+. If you want to break it down more, or the skill requires something more nuanced than a simple Success/Fail, that was the reason for the optional tables that introduced the 111 success (with 91-100 being "near" success). But I'd still argue that the things you have mentioned, while all valid issues, are more of a problem with consistency rather than a bevy of rules. D&D is certainly the most egregious example of a rules-heavy system, while one of my recent favorites, Castles & Crusades, is incredibly rules-light. RM is by no means on the high end of rules-heavy systems and I don't think it's overly complex...
at it's core. But, given the plethora of skills and the cornucopia of optional rules, even presented in RMC, I can certainly see why it is perceived as such.