I was doing some consulting on a small game that I can't really talk about, but there's a lot of customization in the game. I was discussing with a friend (the mysterious Mr. G. Phantome) how we were going to balance this thing. He imagined a future player who works really hard to make his guy into a samurai. He has the samurai helmet and armor pieces, and a decent plasma sword and everything. And then he meets...
Football Helmet Clown Shoes Robot. The terror of the metagame, this guy destroys the samurai guy. Why? Because even though the samurai looks cool, he basically sucks. The mishmash, discordant eyesore of Football Helmet Clown Shoes guy is just way more powerful. We imagined that the forums are full of outraged players, complaining about this, but they don't really grasp the game system. They keep calling for the football helmet to be nerfed, but they don't understand that the clown shoes are what really powers the build, in fact, it's really just the left shoe.
A new player starts playing and asks what pieces he should be looking for. The experienced players tell him "Dude, you want Football Helmet and Clown Shoes, for sure." The sad new guy asks if he has any other choice. "Caveman is barely viable," says the expert. "Flower hands can win too, but loses to everything else so good luck with that."
The point of all this is just that a customizeable system can have aesthetics at odds with balance or power. You have to look out that for the problem that actual experience the player will have with the game is encountering absurdly bad-looking characters. Imagine a fighting game, for example, where one guy is wearing some ugly safari outfit and the other has a huge hat that covers half his character and some enormous robe that covers the rest, so you can hardly tell who anyone even is. Yeah it's fun to customize, but some careful art direction is in order.
Magic: the Gathering
Later, for reasons unrelated to the above, I was scrolling through some images of Magic: the Gathering cards. By chance, I happend to come across the card Donate. I asked Mr. G. Phantome if he knew about how Donate (a seemingly terrible card that gives the opponent one of your cards) was actually part of one of the most powerful and dominant decks ever. He said he was not aware. So then I showed him Illusions of Grandeur, the other part of the combo. I told him to just LOOK at the cards, not even really read what they do, but just look at them side by side, and think about how they were the terror of MTG for a long time.
"Oh my god!" he said. "It's Football Helmet Clown Shoes Robot!" These cards are absolutely awful looking, and of all things, THESE were the focus of the game at that time. This serves as a warning to anyone who would allow customization: at least try to make it so that whatever ends up getting all the attention looks half-decent. Yeah I know that can be hard though!