i always thought scenarios complement rules and vice versa.
I agree, but there is no such thing as "rules" and "scenarios".
You really need to define the terms if you want them to be meaningful. I do when I teach classes on wargame design. My defs are not "the" defs, but they allow people to have meaningful discussions without too much ambiguity or too much "that's not a rule, its …".
F'r'ex, "too many tables" could be rules, scenario, or both.
You could always argue that the combat system having too many things going on that are not relevant to advancing the interest points of the scenario is a scenario design problem (picking the wrong rules). But the way I look at a scenario, it should be as independent of the "rules" as possible. The two should only interact in the specification of "artifacts" – units on the board and terrain pieces – which should be easily convertible from system to system. So for me, it would most likely be a rules, not a scenario problem.
For INLGames rules, QILS, instead of having tons of rules and charts, it has a flexible framework that you specify for scenarios.
F'r'ex, There are no "terrain types", but instead a "terrain system". So the "rules" don't have all 126 different types of snow defined, just a way for you to spec out the type of snow you want. This transforms the cognitive load and admin overhead for terrain. Instead of "this is crunchy snow" and "crunchy snow is -2 movement when looked up in the terrain table", the scenario just has "this snow is -2" and you only need to retain that one but for the duration of the game.
If you only have a handful of terrain types (4-8) per scenario, this is pretty easily covered when talking through the scenario prior to play. If you do need all 126 types of snow for your scenario, then you probably will need to write them down and have tables available. This should not be "too much" for the scenario though. If you "need" all 126, then the differences should be relevant to the scenario. Doing things that immerse you in the cognitive space of the scenario doesn't slow the game down – it is the game.