When you talk about placing it in different eras, what you are really talking about is technology level. Technology level really just means changing the equipment that is available to characters. And when you change the equipment, what you are actually doing is altering the stats that characters get from their equipment.
While this is not going to negatively affect 80% of the classes in the game since they are not particularly reliant on equipment in order to perform (basically all spellcasters)... or the equipment they are reliant on is so basic that you are more or less going to have it available in any setting from stone age to futuristic and can probably take massive advance of future tech (rangers, & rogues and likely barbarians)
The trouble comes with the Fighters and the Paladins. (Clerics too, but to a much smaller degree.)
Basically think of what you are doing as the same thing as slapping all characters who are reliant on heavy armor with a -2 to -4 AC penalty in the low-tech settings or giving all characters who use ranged attacks a +1 or +2 to all damage rolls in the high tech setting and I think you might begin to see how this affects things.
I think one problem with changing the equipment that is available is well.... Dexterity is already the god stat and changing the technology level just makes that all the worse.
Let's say because you are doing an ancient setting, suddenly all the heavy armors aren't available anymore... the best you are going to do is hide armor?
Well then, unless you are a Barbarian who gets an unarmored bonus, it is pointless to even try to build a Strength-based character. Your defenses are going to require you to have at least 14 Dexterity. Every Fighter or Paladin is going to have to be a Dexterity based one or well.... just be well behind the power curve.
Or if you advance the technology level and now you have guns that are simply more effective at doing damage than bows or crossbows and easily on par with any melee weapons? Well, at that point melee becomes a completely subpar option and you are better off boosting your Dexterity so you can take full advantage of the best weapons in the game.
I mean, unless you make ranged weapons Wisdom-based instead of Dexterity figuring the stat that governs Perception is more applicable to using ranged weapons than one's ability to jump and roll around and juggle objects. But assuming you keep Dexterity as the stat that governs both your dodging ability AND your accuracy with anything that weighs less than 10 pounds.... and, either way, again-- Strength becomes pointless beyond the minimum level you need to carry your equipment (and since you aren't going to be wearing armor-- that reduces the amount you need to carry anyway).
The classes in the game are balanced with the expectation that characters are going to have an specifically available set of equipment with particular stats. You change the available equipment options and either decrease the AC bonus one is allowed to purchase or increase the damage of ranged attacks and the system, while it doesn't collapse entirely, is simply no longer going to be balanced. But, again, it is worthwhile to point out once again that only two classes in the game are negatively affected.... Of course, granted, races that grant bonuses to Strength are also seriously negatively impacted by this unless they are barbarians in the low-tech setting and always in the higher tech setting. Half-Orcs are absolutely the most screwed except as barbarians in the low-tech setting with Mountain Dwarfs also becoming a seriously underbalanced option in either setting.
