Team Power is a reasonable comparative measure between two offenses, or between two defenses. What it is not good for is comparing offenses to defenses.
In this game, the attacker has a very strong advantage once team power gets past about 2500-2700 points. There are several reasons for this, and this excellent post by @NPNKY is basically the bible for this issue:
A good defense is one that will win when boards are marginal (or worse) for the attacker. If an attacker has a good board, only insufficient availability of the right kind of hero for a team, poor team selection, or poor play will cause a loss.
This is why most advanced players focus more on building a strong, diverse roster of attacking heroes, rather than squeezing every last drop of juice out of their defense team. There’s nothing you can do to keep your defense from being beatable, but there’s a lot you can do to make sure you’re winning fights on offense as often as possible.