Except for the fact that the scout is way overpowered now. He can slip past sentries unharmed, and essentially stop an uber team dead in its tracks. Scout is the ONLY class that can do ANYTHING to an uber team (if you don't count the Pyro's minor ability to blast them back).
Assuming every new weapon has a balanced downside is to assume that Valve is infallible. The Scout was already really powerful in the right hands, and now anyone who unlocks the new weapons just has more tools with which to dominate.
This was expressed already, but I think giving players a means to unlock new weapons gives them a leg up on those who haven't yet unlocked them. People are always going to just go to an achievement server and go around the ridiculous achievements that way anyway, and the dedicated players who do this will have all the new stuff in a couple of hours. Those who don't play so much just have to suffer. Maybe if it were something where everybody gets the weapons off the bat (no pun intended), and maybe had the option of unlocking reflective gold or platinum skins or something, that could be cool. It would show others that someone unlocked those achievements without actually disrupting the balance of the game.
BTW, I don't know if Valve just lied, or if their method of detecting who used Achievement Manager to unlock their stuff just doesn't work, but I unlocked all the stuff the other day with it, and I still have it. I got fucking frustrated that I had to go to crazy weird lengths to gain these achievements when I just want to play the game like everybody else. It's ridiculous. I did unlock a good number of the Scout's achievements in-game, but the rest I just got fed up and tried the unlocker. I still have my achievements, and my unlocked weapons. I even finished getting all the achievements for the Heavy since I hadn't bothered to sit around doing that and only had 8 by accident, so now I have the gloves and stuff.
In short, these achievement things are interesting but really stupid. Why every game has to have them these days is beyond me. I guess it's to artificially inflate the replayability of the games for those who are OCD like myself.