FormerlyHemlock
Hero
You can fairly categorize most games as emphasizing these three elements: 1) Combat, 2) Exploration, 3) Social Interaction. Each game will involve those aspects to differing degrees, and it's fair to say one campaign may emphasize one of those things at the expense of the other two, or two of them at the expense of the third. But I think you exaggerate when you say there are millions of scenarios you'd need to account for. Not really - most things come down to one of those three elements, and you can talk about generalizations concerning each ability, option, and spell concerning those three aspects of games. I mean sure, we know if you are facing only fire demons then your fire spells might not be so useful - but you also know most games are not just facing only fire demons. You can generalize, which is what such guides are about. And the generalizations are useful - they are a stepping off point for discussion, a baseline for new players or players new to a class to look at, they can be fun to read, there's all sorts of useful stuff with these guides. It's why they've been popular threads for decades now.
You need to account for high magic vs low magic worlds; rest frequency; whether your campaign takes place in cramped dungeons vs. open spaces (i.e. is 60' range on an ability a significant limitation?); party size; whether your DM tailors combats to PCs; whether your DM leans more on foe quantity or quality (do you fought hordes or boss monsters or both at high level--affects the desirability of nova tactics); whether your DM metagames monster tactics or RPs them; whether your DM and/or fellow players are GOOD at tactics when they try to be; availability and affordability of auxiliary troops; treasure accrual rates; whether transport and logistics are a concern for your DM (affects both minions and the desirability of spells like Goodberry, Mending, and Fabricate); whether poisons can be bought or made; and... that's all I can think of in sixty seconds. Oh, and how fast your fellow PCs are, and whether tactical recon is a thing for your fellow players. And whether your DM follows DMG encounter-building guidelines or lets you fight Iron Golems at first level, and how likely he is to make up new abilities on the spot (like letting Tarrasques and Iron Golems their rocks like fire giants when he sees first level characters killing them). Does your DM grant kill XP, and does it have to be an actual kill. Okay, more I think I'm really done for now.
Trying to generalize over all those factors is intractable. All you can do is state some assumptions and then explore certain spells within those assumptions. E.g. "If your game features open terrain and sandboxing, Expeditious Retreat and a cantrip enable power leveling on melee-centric monsters like Iron Golems." "If minions are a thing, Fabricate can give them all AC 18 (chain mail and shield) for just the cost of some scrap iron and charcoal."
Intractable.