Prior to "Call to Arms", starfleet's shields were ineffective against dominion weaponry.
I was referring to after "Call to Arms" - the dominion heavy ships or cardassian orbital platforms (beefed up by dominion tech) were repeatedly shown brutalising starfleet/klingon/romulan ships. Despite their shields working efficiently - the dominion weapons were't slipping around the shields by technobabble, but punching right through them.
Of course, large caliber starfleet weapons (DS9's weapons, defiant's, etc) were also shown easily destroying dominion fighters.
No hiding behind one's shields while reciting an inspired speech about federation values in this war.
Unless you want to get blown out of the sky in a minute, tops.