Agility is the basis for initiative rolls, and since combat doesn’t last many rounds in OL, this creates a significant advantage over other combat attributes like might or energy. Agility is also the attribute which grants access to the highest number of banes (tied with entropy).
Besides flavor, are there any reasons to build a character with main attribute other than agility?
Easily a million (slight exaggeration of course, but really, only slight).
ANY build is viable and effective in OL. Min/Max 'ing just doesn’t do much in OL. Initiative isn’t all the big really.
This thought has been brought up many times before about agility. It seems like it on paper, but actual play is actual play and will give you a better feel.
Anything can be situational to whether it is actually good or bad, and any build can be countered.
Having high initiative is often no benefit by itself. Going first is nice, but if your party can’t or won’t follow up because they rolled low on initiative, than it can be a detriment, so I don’t think it’s that big of a deal.
Also, counting the number of banes an attribute gives you access to isn’t all that great as a metric, because in most cases you can only apply one per round anyway, so what use have the other twelve at that moment to you?
Agreed on high initiative not always being a good thing. In fact, there’s a whole mechanic, Waiting To Act, that serves the purpose of letting you make your own initiative even lower completely voluntarily!
I was asking for advantages other attributes have over agility, not for the number of possible characters 
Even if the other builds are almost as effective as one with high agility, the small advantage still remains. And if there is no compensation for other attributes, they are strictly dominated by agility (even if only in combat). So the only incentive to play a different character would be flavor, which is a strong reason in itself.
Do you mean that such for instance a high agility character has a weakness in toughness or resolve, and thus the GM can throw some influence/entropy guys in to take advantage of this? Or a surprise situation, in which the agility character doesn’t get to be high in the order due to being surprised? Or do you mean something different with “countered”?
I don’t think it can be a detriment,because, as Crazy_Mac points out, you have the possibility to wait. By having a good initiative you gain the choice to do something directly or take your turn whenever you see fit. Having this choice is always better than not having it. Also, if everybody in your party had a high agility, they would also stand a good chance to be high in the initiative order.
Would it make sense to let players roll for initiative using another attribute than agility, if they can justify its use? As in “My character is a very good wizard and having energy 5 lets him cast spells super quickly” or similar
That’s perfectly reasonable, yes. Personally, I use Agility as the default but will occasionally ask for players to roll something different for initiative depending on the situation: maybe Perception to see how quickly they work out what’s going on, or Deception for the inverse where they’re trying to hide their intentions. I’d be wary of letting players always decide their default, but as was mentioned above it’s not too big of a deal so as long as they stay away from the nonsensical you should be fine.
To your initial point, I do agree that Agility is probably stronger than the other attributes, but I don’t consider it a problem. You can’t use Agility to lift a portcullis, or rig an explosive, negotiate a good price or catch the shopkeeper in a lie. As @Great_Moustache said, pretty much any build can be viable in OL; as long as you have reason to invest in other attributes instead (which you definitely do) then it’s not overpowered even if it’s the strongest on paper.
And that’s what I was answering. Everything is situational to how useful or powerful it can be.
In fact, more effective given what the campaign is, what the world is like, and what a given encounter or difficulty is like
No, as in, if you build a character, I can then build a character that can take them out pretty easily. However, yes, a GM can make an encounter challenging, and give moments for any build and character to shine.
It may or may not be a detriment, but the whole point is, it is not a big deal at all. There can be a few niche situations where going first as a huge bonus to it, but like above, that can be situational.
@SamWilby already answered this, but I’ll echo him here. Plus, via Attribute Sub you can use other attributes. Personally I’m in favor of just using a d12 for initiative! I use the default in my games though.
And like so many things, you’ll see this in actual play. The sheer number of viable and useful builds out there is huge. Given that dice explode, and can equally roll low, anything can happen in an actual game. Like @VanGo said, simply having access to a lot of banes isn’t all that much, b/c you only have so many actions. What it really comes down to is the number of dice you keep and the number of advantage you have to a roll. And anything that helps increase your action economy.
I really don’t think a single attribute is superior to any other. It comes down to the combination of things you choose from all the attributes, feats, perks, and flaws that will determine.