...some players will look at a codex and figure out that X unit just isn't worth it and field it very rarely...
This is what I've come to conclude about the Space Marines Land Raider. For 250 points base, it just doesn't give me the return that 2 or 3 other units that total up to 250 points would get me.
But onto the Tyranid Codex, I don't think that there have been that many drastic/amazing changes done to it. I was fairly ignorant of them until the new codex came out, when a lot of my friends in NJ were raving about it, but from what I surmise about it is that a lot of Elites, HQ's, and Heavy Support choices switched roles, and were modified differently to accommodate certain flavorful lists. When my friend started going on and on about a few units, I made it a point to ask which category they fell under on the Organization Chart, and the majority of them fell under Elites or HQ's. So before worrying about all this uber crap he was going to bring, I did the math, and realized that it's impossible for him to field that many Elite and HQ choices. And therefore, nothing left to worry about.
As for the units that I do know of, the Trigon, the Doom of Malintide (spelling?), and the Monstrous Creature that acts as a "Drop Pod" are the only units I've seen out of the new codex in a game, and this is my analysis. The Trigon, while it can deep strike, should start the game on the board. If it deep strikes, then you run the risk of it mishapping or just not getting to come out in time when you need it (This happened in one of the games I played, and as it turned out, it didn't come into the game until Turn 4, and by then his Hive Tyrants were dead, along with all of his other elite choices, making it the only target 2/3 of my army had to shoot at. It never got to assault, as a result of that).
As for the latter two, they were much more effective in combat. Only downside to either of them is that they are susceptible to Instant Death.
So all in all, I don't think that it's as "broken" as some people said it would be; just different, with a few adjustments. Being that I don't play Tyranids, it's hard for me to truly assess the codex, but as a person who's played against the old and the new one, I don't see many daunting differences.