Over the years I've come to really really not trust florists ability to place the flowers on my wedding cakes. WHY? Many reasons.
What happens in my experience....
1.They don't really know what the bride wants on the cake. I go thru the time of finding out the specific look they want and the florist doesn't know, no one informs them. Even when I call them to make sure they'll have the right amount and type of flower they seem to ignore my conversation and come un-prepared.
I've had so many cakes that looked horrible because of the flowers... I did the M.S. cake that has NO decorations, only a top and bottom border. In the center between each cake gets perfectly stacked roses (angry did this too)....well the florist I had for this one just stacked the flowers loosely into the center. They kept falling out ruining what was supposed to be a perfectly clean lined cake. The look was so loose, totally not similar to the photo. I even brought out the picture of the published cake to show the florist. Finally I consented to let them poke the flowers into the cake (I had all my servings in the back so I was covered), they really messed up my cakes. If they knew what the cake should have looked like, they should have arranged sizes and heights with me then placed the roses into pre-done rings placed on a plate between layers.
I can tell you tons of stories where the florist just totally missed decorated what the bride wanted. How about those god aueful perfect carnation tops that are a perfect rounded mound???, YUK! And then their either too large or too small in proportion to the cake.
2. They decorate the cake last after their done with 'their' part, (the cake isn't their part).... I've had florists that didn't even have flowers left for the cake (which was previously arranged). Oops, they ran out. Then I have to go around and steal from other arrangements WITH their help. Or all they have left is greens for around the base....then1/2 the time it's the wrong type of greens for the situation. Like poofy ferns that hide the whole bottom layer of a small cake or the opposite when your using a stand they provide 4 flat ferns that don't cover the base as planned.
3. Timing. They are either running late and your waiting for them, or your late and they left.
In my brocure I've written this "Flowers can be provided through your florist, although I prefer to place them on the cake myself, to maintain the integrity of your design." I guess I'll see how this goes. I know EVERY, EVERY baker around feels just the opposite. They'd rather do the cake and run, wash their hands of the florals. But I have too many cake photos that would have been beautiful if the flowers had been right, and they weren't. I figure if I want a good rep. then I need to be certain the cake is totally finished according to what the bride wanted.
P.S. That all being said, there are a couple different florists that are GREAT, WONDERFUL where I couldn't possible do it better. BUT they're the minority. I stand back and let those people totally decorate the cake.