I don't recall all the the different ways to get evasion, but let's just deal with the basic classes of monk, rogue, and ranger.
Monks and rogues both gain evasion at level 2 and are worded the same: Evasion can be used only if a monk/rogue is wearing light armor or no armor.
Monks are not proficient in light armor, so I'm a little confused all ready.
Rangers gain evasion at level 9 and change the wording slightly: Evasion can be used only if the ranger is wearing light armor, medium armor, or no armor.
So rangers are better at evasion than rogues?
What if I'm a ranger 4/rogue 4? Can I only benefit from evasion in light or no armor? I feel like that is correct RAW, but then suddenly at ranger 9 I learn how to use evasion in medium armor?
Was this just a balance issue? Rangers get it later so it's better?
It's obviously not a proficiency issue since monks aren't proficient in light armor.
Any thoughts on this? I couldn't find an FAQ or errata on this.
So rangers are better at evasion than rogues?
Yep. They just have to be much higher level to do it.
What if I'm a ranger 4/rogue 4? Can I only benefit from evasion in light or no armor? I feel like that is correct RAW, but then suddenly at ranger 9 I learn how to use evasion in medium armor?
You can only use the rogue's evasion with light armor or less. You can only use the ranger's evasion with medium armor or less. If you're a ranger 9/rogue 4, you can use the ranger's evasion, rendering the rogue's evasion useless.
Was this just a balance issue? Rangers get it later so it's better?
Probably more of a role thing. Rangers are meant to be capable of using their class abilities while wearing chain mail or other medium armor. Nerfing evasion would neuter the ranger; either they'd have to give up medium armor, a staple of the class, or they'd have to give up evasion, another staple of the class. Getting it later might be a consequence of that design; getting it as early as the rogue would be an even bigger middle finger to the rogue, who's supposed to be tops at that sort of thing.
It's obviously not a proficiency issue since monks aren't proficient in light armor.
A monk can wear light armor, though, even without proficiency. Alternatively, they could take Light Armor Proficiency as a feat or dip a level in a class proficient with light armor. In any of these cases, their evasion still works, though some of their class abilities may not.