My favorite beers are in the brown ales to stout spectrum. If I can get the sweetness balanced, I probably will be satisfied with the results.
There's all sorts of beers you could make with a combination of darker malts but throwing them all together and hoping for the best is one of the best ways to make a recipe that tastes like brown rather than something you really want to drink.
I think what is meant is that the beer will taste muddy rather than like a brown ale.
A brown ale is mostly base malt with ~10%-15% crystal/color malts for color and flavor.
The other thing to think about is that crystal malts add unfermentable sugars to your brew so the extraction isn't the only thing to be concerned with. at 45% you might well end up with a lower attenuation than you would like.
But that said, it's your beer.