They aren't visually distinct enough. Individually, they are quite exceptional and I applaud your technical prowess completely. However, putting a whole bunch of them next to each other, and I honestly have a hard time telling the apart. If you put all the Final Fantasy Tactics characters on a sheet like that, there would be an immediate and obvious distinction between each of the units, and even when a bunch of them are clumped together on the board, you can still separate them at sight. With yours, I'm having trouble figuring out which sprites are supposed to be the same character classes, and probably couldn't connect the fronts with the backs with a map.
My recommendation is to not put as much emphasis on what they are wearing, and instead give each unit type a one or two color scheme - never have two hats the same color, since that is where the identification is going to be most focused. And when picking your colors, you've got a very muted low saturation thing going on, which is going to make that difficult. You want their primary color to leap off the screen at a glance, as it is absolutely the most fundamental and important aspect of their character. Your composite images shouldn't look like a mashing of blues-purples-oranges, but you should be big self-organizing chunks of a single color.
It may seem counter to what you want to do, but make a little sample image where you put maybe nine guys standing in a square on your isometric map. If you can't instantly tell which units are which, as I suspect will be the case, you need make special effort to change that. Go check out the graphics for a Nippon Ichi game or any other Tactics game and you will undoubtedly see how important distinct coloration is to these types of games.
