Like others have said - any army can compete and the power gap isn't as wide as it is (or at least
was) in
40k. This article is a bit outdated - it's before the Ogre and
VC update - but still pretty accurate:
http://darkfuturegaming.blogspot.com/2011/05/warhammer-fantasy-power-rankings.html the guy himself even says it's his own opinion based on big+small tournaments and forums, and and that the balance of power is closer since 8th Ed.
But again, it's closer than you think, don't think 15th has no chance to beat 1st. Wood Elves are the consensus worst army in the game (as far as I can tell, that's how the internet views it). But they placed 2nd at this year's Bay Area Open and were in the Final 6 at Ard Boyz West last year. So they can still get the job done, given the proper list, tactics, and general. ...and little luck.