Sliders sucked. I'd rather they just gave us more national ideas.
Centralisation/Decentralisation the most obviously terrible. It implied that centralised states were automatically better than decentralised states and there really wasn't enough of a penalty for going beyond your government type's cap for it.
Aristocracy vs Plutocracy wasn't so annoying in concept, but kind of stupid since Aristocracy mostly just made stuff cheaper while Plutocracy gave you more money. Cavalry also sucked if you had western tech so you were generally better off with Plutocracy even if some countries kind of favoured aristocracy and the leader bonuses were nice.
Serfdom vs Freesubjects was almost as bad as centralisation, except it pretended it wasn't so one sided by giving serfdom cheaper galleys. Galleys were useless anyway unless you only fought in the Mediterranean and had enough coastal provinces to make 500 odd of them. But the real problem was the freesubjects actually have you better tech bonuses than innovative did so you would really never take serfdom, even if you really needed reduced stability costs it was better to just reach higher tech levels easier and more national ideas and government buildings so you could reduce stability costs way more than serfdom did.
Innovative/Narrowminded was actually sort of okay. The fact that you could make up for the tech penalties with freesubjects, the boost to colonies and just how hard it was to get missionaries otherwise actually made narrowminded kind of viable. But I still always went with innovative because at its heart this is another slider that basically says "do you want to be an idiot?"
Mercantalism/Free Trade was the worst one, though less obviously so than Centralisation. For most countries it literally meant 'wait 50 years before you can even thing about getting any trade income. You could get rich of mercantalism, such conquering every COT out there, but you were better off doing things the easy way.
Offensive/Defensive was actually not so bad. At first I just always picked offensive, but I think defensive is actually the better one. It isn't even really defensive, since it makes you better at siege in general and siege is such an important part of the offensive in this game.
Land vs Naval was kind of cool. But at the same time by not sucking as much as some of the sliders it showed how terribly the system was designed in general. But it was really probably another no brainer towards naval in the end, since unless you were Russia, you couldn't be a global power without a navy and EU3 is a game about becoming global. Naval also gave you more money so that made up for the more expensive land troops while land didn't even make every land unit type cheaper, artillery being covered by defensive instead and all.
Quality/Quantity wasn't offensive like free subjects or free trade but it wasn't really that interesting either. It was also kind of counter intuitive since small countries needed quantity more in order to make up for their weaknesses while large countries with low tech probably needed quality more to make up for their weaknesses. There was also a disconnect with all those events forcing you to go quality which made it seem almost like a centralisation no brainer style decision even when it wasn't.
I never tried doing a world conquest, so maybe the slider choices do change if you're doing that, but I find world conquests boring so the fact that they make sliders a harder choice just makes sliders more boring to me.
On the other hand I thoroughly disagree that it is hard to imagine other systems being worse, the "laws" system of CK2 is definetly worse.
How so? Crown Authority and Centralisation are basically the same. All the other sliders from EU3 would basically be meaningless in CK2, a game that has no naval combat or trade, little innovation and the going away from serfdom and aristracy would be shooting yourself in the foot but is at the same time better represented by your ability to create lord mayors and doges in your kingdom.