I think the big issue is the "fun" factor with Fixer, and games where he really NEEDS other heroes to allow him to draw more cards in order to get what he wants when he wants it. While mechanics is an aspect of that, the main frustration is that it isn't terribly fun waiting for the right card to make you heroic. Also, limiting his card accelleration to Toolbox and an extra draw when he doesn't use powers/play a card and not being able to search his deck for Tools makes it extra embarassing when other heroes have to lend a helping hand. It all just feels a little...underwhelming.
With Legacy, how often does it feel like he is compromising the good time of the players? How often do the heroes have to cater to his lack of card draw? How often do one of his turns feel wasted with his Galvanize innate power? I think this is why people don't often bicker over Legacy being overpowered or whether Haka or Ra are better at dealing damage and which one is more efficient. It's more a factor of how "heroic" each individual character feels. Adept and Visionary master the board, Wraith pulls every gadget known to crime-fighting out of nowhere, and Nightmist blasts big bads with insidious energy beams powered by her own soul (with plenty of opportunities to pump her HP back up for another go). Legacy, though potentially overpowered, helps everyone feel more heroic with his deck design purely through being on the field.
Fixer, however, has many a situation where he relies on others to get him what cards he needs to have for a given situation. When he has what he needs for a situation, he is a force to be reckoned with. I definitely love Fixer when he's doing what he needs to do to contribute, but those games when he doesn't really can be a let down. With almost every other character, they have multiple avenues of card accelleration and/or retrieval cards. Even if those cards aren't drawn, the very potential of being able to use them yourself is rather comforting. With Fixer's Tools, it's either a lucky Toolbox draw or a lucky draw from another hero's card accelleration.
I understand that there have been many attempts at houseruling his (and more recently, Expatriette's) deck to see about fetching more cards or allowing for more card accelleration. With some of those attempts, it has apparently made these characters somewhat imbalanced, even overpowered. I find that to be a little disappointing, because I love it when Expat can mow down minions with a SMG or Assault Rifle, and Fixer with Alternating Tiger and Dual Crowbars is brilliant against Apostate. The idea that making those opportunities a little less random will ruin the game's balance is a real let-down. I get that a lot of people love Fixer and Expat's deck as-is, but the idea that there is just no room for searching their decks more reliably or drawing a few more cards makes those games with a crappy hand really sting.
My frustrations with Expatriette and Fixer have been expressed in terms of mechanics, and I don't think that my (or many other Sentinerds') observations are completely unfounded. But none of that would really matter to me if not for the fact that I feel so useless when I can't deliver on a Tool or Style to beat the villain and card accelleration needs to go to someone else. It's the lack of self-sufficiency that really disappoints me with characters like Fixer, because sometimes my teammates need to help someone out that isn't me. Rarely do I get that same disappointment of being utterly helpless with characters like Legacy, Tachyon, or Chrono. I still get bad draws, but the idea that I can support the team with a Galvanize, draw a few more cards with some Fleet of Foots, or pull out The Masadah with a Displaced Armory, even if bad luck doesn't allow for it, really softens the blow of defeat.
- which is why I don't enjoy Newest Legacy as much but love Greatest Legacy. (And it honestly surprises me I still enjoy playing Legacy after more demos playing him than I can count!)