I just read an interesting piece that I found through TauOnline.org entitled "Is Warhammer Balanced?" This blogger has taken tournament data for both Warhammer Fantasy and Warhammer 40,000 for a particular set of tournament data and determined the mean score for each faction in order of codex release date. The purpose? To determine, as best as can be done with the data at hand, if any power creep has occured from codex to codex. Power creep, the ramping up of in-game power from one supplement to another, is a common issue in any sort of ongoing expandable game, whether it be CCGs, RPGs, or wargaming. The third edition of Dungeons and Dragons, for example, became increasingly prone to power creep as more and more books were released, with later character classes completely outshining those from the core rulebooks.
What the blogger in question has found reinforces some already-accepted assumptions, especially in regards to Warhammer Fantasy. Specifically, the answer to the posed question is, "No, Warhammer is not balanced." Fantasy in particular had noticeable power creep in the newer codices. Warhammer 40k, on the other hand, had a much smoother, shallower increase. Granted, it doesn't mean that one game's factions are more closely balanced than the other; Fantasy and 40k play very differently, so options are available in the latter game that aren't in the former, and those options can even the playing field somewhat.
Also, while the most popular armies in Fantasy tended to be the best performing, that did not play out in 40k. Space Marines were the second most popular army, but came in with the lowest mean score. Does this mean that there Space Marines are the weakest army? Or does it mean that there are a lot of really bad Space Marine players out there? I'm leaning towards the second explanation; they are one of the most popular starting factions in the game, so a lot of first-time players may be giving them a go. Unfortunately, the article (and the data it's based on) isn't clear as to whether these games were played before or after the new Space Marine codex came out. If not, the second-newest codex - Orks - comes in as the most powerful. That doesn't necessarily bode well for power creep, but consider that the last pre-5th codex, Daemons, sits right in the middle of the pack. However, if they are post-Marine-codex games, then power creep doesn't seem to be apparent (although again, the relative skill levels of the players may skew that). I'm not too worried, though. The next codex is Imperial Guard, and I'm highly suspicious that the IG are suddenly going to be the killer army du jour.