It's difficult to tell how hard that treaty was on anybody, because we never learned its contents, or any of its consequences. All that was ever said was that the fighting ended. Nothing was said about, say, the Jem'Hadar withdrawing from the planets they still controlled (if any), or the Cardassian Union being chopped up between the winners, or the Federation agreeing never again to venture to Gamma - all possible elements in the treaty, but unfortunately inaccessible to us.
The Breen are not sitting down at the table where the Founder signs the treaty. They stand next to Feds, Cardassians, Romulans and Klingons, while just the Gamma species of the Dominion are sitting down. Are they among the losers, or the winners?
Cardassia switched sides at the last moment - and so did the Breen, for all practical purposes (their generals walked out, suggesting they were planning something completely dissimilar to the official "we're needed at the front lines" party line!). Perhaps they reap the rewards of that decision now? If placed on the same line with Cardassians, they emerge as the definite winners, as their old enemies are now badly hurt by the war while no Breen world was touched AFAWK.
At a deeper level, we're left wondering who won the war. The Dominion got Odo back, which may be the only reason they stopped fighting. And both sides may realize that had they not volunteered to stop, the Dominion would have won. Perhaps not that day, but certainly a few decades hence, when the reinforcements arrived. (And perhaps the war will continue when they do arrive, via the slow route...)
The treaty might well have been one of conditional UFP surrender to the Dominion, rather than vice versa. The one known condition being that the Female Founder would "stand trial" and "accept responsibility" - but the courts might not dare give any other verdict but "not guilty"...
Timo Saloniemi
