It's balanced, what is your problem?
In my opinion this game is indeed very balanced, and the crypts are by far not the only way to get a dozen 'brilliant's in no-time. I agree on their usefullness since they level up very fast and through their special abilities like freeze or bouncing shots have a one-shot stopping power unlike all other towers, but if played right, the temples can beat that without problems. with a bit of strategy, a combination of scorching auras at the entrances (in high levels this kills all ninjas instantly, plus it slows down the enemies right from the start) and evil eyes together with a few dens and crypts near the gems kills anything before it comes even remotely close to them. I have played crypts and temples to near perfection and so far it has been a great (and time consuming...) challenge, but very much managable. Since i am still playing the Dens to try out their strengths and weaknesses i cant say much about them, but the ballistas have been very reliable in killing ninjas so far, and the crit towers can kill any single enemy fairly fast, too. I can't say much about cross-tech strategies, since I haven't tried it yet. But each 'pure' tree can be used in ways that can wipe the Heroes off the field.
Additionally, about the 'beloved' crypts everyone seems to be so into: with them and even a bit of bad luck, you'll have 5 assassins stealing your gems pretty fast. i really do prefer constant-fire temples over all else, they may not be as fast in killing everything as the other towers, but they also do not have real weaknesses. plus, in later levels the game becomes VERY hard without them. Might as well level them right from the start.
finally, what's the point in debating which tree or combination is most useful? i enjoyed the game, and really: if you are good enough to beat the game, it shouldnt matter which tech tree you take. After all, it's always about the right tactics and a bit of luck.