Logia types seem overpowered. Excluding the fact that they are practically invincible to anyone who is not a Haki user (if they can reflexively turn into their element), they have many more advantages:
Think about how outrageously powerful some of the Logia types could be if they were creative. Almost every other strong character in One Piece (Devil Fruit user or not) is strong because they were either creative as hell, had sheer strength and technique (Zoro), are Haki users with some sort of fighting ability or a combination of all three (of which Luffy is a perfect example). My point is, they all had to be creative or strong and skillful, with Haki to boot (sometimes). Let's be honest, most of the Logia types are so powerful because of their Fruits, I am not denying that their own, personal skill didn't attribute to some of their strength, but let's be real; you've got people like Enel, Kizara, Sakazuki and Ace. Pre-timeskip Ussop would be a formidable opponent with any of these Devil Fruits. It takes a lot of Logia types no physical training, and very little creativity to be a dangerous person to face. If these guys had creativity and didn't completely rely on their Devil Fruits' sheer power, very few people in the OP universe could beat them. Sakazuki could make his opponent's entire stepping ground a field of lava, he could simply make lava "minions" like mannequins made out of lava that could smash into his opponents. I'm sure there are more things he can do, I just made that off the top of my head. Anyways, there seems to be so much they could potentially do, that could wreck their opponent, but they are too comfortable in their abilities to even fathom needing more variety to beat someone.
A Logia can produce limitless amount of their element, this screams overpowered. Someone as formidable as Hawkeye Mihawk could be rendered useless to Ace if he simply chose to send an island-sized fire ball, after all Logias can produce an endless amount of their element. Even if Hawkeye is very fast, Ace could simply use the fire ball if he neared him or if he moved out of sight, and nuke the entire area with it. I doubt even Hawkeye is fast enough to get out of the destruction the attack would cause. Or take another hypothetical situation, Enel verses Shanks. Shanks is one of the strongest characters in One Piece, but if Enel uses his supercharged mantra plus his ability to travel at the speed of sound, he would own Shanks within moments.
This is a bit of a combo of my two points above, if they can control and manipulate their element in any fashion and use it limitlessly, couldn't they just create a repetitively powerful attack? Okay, some of them would need time for these attacks, but what about the extremely powerful ones that don't? Enel can easily send 200,000 volts into someone. I realize this won't immobilize some of the heavy hitters, but it would slow them the fuck down after he hit them with it for a second time. Kizara could just lock someone in a light cage and shrink it until they combusted inside, Ace could lock someone in a fire sphere and shrink, I am going off on what I read on Logias, that they can manipulate their element in any way, so they don't have to touch it to control it. Why hasn't anyone done this?
I would bring up other situations that a Logia could have dominated in a split second if they had simply thought of said tactic, but that would take too long. I know some Logia types are more restricted verses others, and don't follow the exact definition, but still most aren't. Logia types could easily destroy islands and kill strong people in moments, it is ridiculous considering Paramecia types and Zoans being on-par with them, some but not many. If we follow the Logia's definition, Logias should be dominating everyone, only challenged by other Logia types. It is the only way (that I know of) in which a weakling or complete idiot can become one of the most dangerous people in OP, maybe the idiot would die, but think about how easily he could decimate a town. And the weakling is worse, they could strategize with power and beat the shit out of anything. I know even the extremely powerful Logias can easily lose, but a lot of that is up to circumstance and they still have a better chance at winning unless they are fresh newbies.
Seriously, everyone else has to work their ass off to just compete with Logias, and they use almost non of the creative end of their powers and just rely on its sheer destructive nature, and I think Oda knows this. Oda probably knows almost no Logia uses their abilities to its fullest, not even a tenth of its potential, and is still extremely strong. It might be a long running joke.